1. Personal sin: offense against God, disobedience to the Divine Law
Personal sin is an “action, word or desire contrary to the eternal law."  Sin is a human act , since it requires the exercise of freedom,  and is expressed in external actions, words, or internal acts. Moreover, it is a human act that is evil . That is, it is opposed to God's eternal law, which is the first and supreme moral law, and the foundation of all other law. In a more general sense, sin can be defined as any human act opposed to the moral norm, that is, to right reason illumined by faith.
Sin therefore involves a negative stance with respect to God and, in contrast, a disordered love for oneself. It entails aversio a Deo et conversio ad creaturas , a turning away from God and a disordered turning towards earthly goods. The aversio of sin does not necessarily represent an explicit hatred of God, but rather the distancing of oneself from God, as the result of placing an apparent or finite good above mankind's supreme good (conversio ad creaturas). St. Augustine describes it as “love of self that reaches the point of contempt for God."  “In this proud self-exaltation, sin is diametrically opposed to the obedience of Jesus, which achieves our salvation (cf. Phil . 2:6-9)" (Catechism of the Catholic Church, 1850).
Sin is the only real evil. Other evils (sickness, for example) in themselves do not separate us from God, although they are certainly the deprivation of a good.
2. Mortal and venial sin
Sin can be divided into mortal or grave sin and venial sin (cf. 1 John 5:16-17), depending on whether one loses God's grace totally or not.  Mortal sin and venial sin can be viewed respectively as either the death or the sickness of the soul.
“Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent."  “With the whole tradition of the Church, we call mortal sin the act by which man freely and consciously rejects God, his law, the covenant of love that God offers [ aversio a Deo ], preferring to turn in on himself or to some created and finite reality, something contrary to the divine will [conversio ad creaturam]. This can occur in a direct and formal way in the sins of idolatry, apostasy and atheism; or in an equivalent way as in every act of disobedience to God's commandments in a grave matter."  Grave matter : this means that the act is intrinsically incompatible with charity and therefore also with the unavoidable demands of the moral or theological virtues.
Full knowledge (or advertence ): that is, with the realization that the action carried out is sinful, namely, contrary to God's law.
Deliberate (or full ) consent of the will: that is, one openly wills this action, which one knows is contrary to God's law. This does not mean that, for there to be mortal sin, it is necessary to wish to offend God directly: it is sufficient that one wishes to carry out something gravely contrary to the divine will. 
These three conditions must be fulfilled simultaneously.  If one of the three is missing the sin is venial : for example, when the matter is not grave, although there may be full knowledge and full consent; or when there is not full knowledge or full consent, even if the matter is grave. Logically, if there is neither advertence nor consent, the conditions are lacking for calling the act sinful, since it would not be a properly human act.
2.1 Effects of mortal sin
Mortal sin “ results in the loss of charity and the privation of sanctifying grace, that is, of the state of grace. If it is not redeemed by repentance and God's forgiveness, it causes exclusion from Christ's kingdom and the eternal death of hell" (Catechism of the Catholic Church, 1861). 
When a person has committed a mortal sin, and remains outside the “state of grace" (without recovering it in sacramental confession), one must not receive Communion, since one cannot wish at the same time both to be united to and alienated from Christ: one would commit a sacrilege. 
By losing vital union with Christ through mortal sin, one also loses union with his Mystical Body, the Church. This person does not stop belonging to the Church, but is as it were a sick member, which harms the whole body. It also brings harm to human society, since one ceases being light and leaven for others, even though this may go unnoticed.
Through mortal sin any acquired merits are lost (although these can recovered on receiving the sacrament of Penance), and one loses the capacity to acquire new ones. This person becomes subject to the slavery of the devil; the natural desire to do good is diminished, and disorder is unleashed in the faculties and affections.
2.2 Effects of venial sin
“Venial sin weakens charity; it manifests a disordered affection for created goods; it impedes the soul's progress in the exercise of the virtues and the practice of the moral good; it merits temporal punishment. Deliberate and unrepented venial sin disposes us little by little to commit mortal sin. However venial sin does not break the covenant with God. With God's grace it is humanly reparable. 'Venial sin does not deprive the sinner of sanctifying grace, friendship with God, charity, and consequently eternal happiness' (John Paul II, Reconciliatio et paenitentia (December 2, 1984)" (Catechism of the Catholic Church , 1863).
God forgives our venial sins in Confession and also, outside of the Sacrament, when we make an act of contrition and do penance, being sorry for not having lived up to his infinite love for us.
Deliberate venial sin, even though it does not separate us totally from God, is very unfortunate and damages our friendship with him. We should have a “horror" for deliberate venial sin. For a person who truly wishes to love God, it makes no sense to consent to small betrayals simply because they are not mortal sins;  such an attitude leads to “lukewarmness." 
2.3 The “fundamental option"
The moral theory of the “fundamental option,"  which rejects the traditional distinction between mortal and venial sins, holds that the loss of sanctifying grace through mortal sin (with all that this entails) compromises a person so fully that it can only be the result of an act of radical and total opposition to God, that is, of a fundamental option against him.  The defenders of this erroneous opinion claim it would be almost impossible to fall into mortal sin in the course of our daily choices; or if this were to happen, to recover the state of grace through sincere penance. A person's freedom, they claim, in its ordinary exercise of choice would not be able to determine in such a unique and decisive way the state of one's moral life. And since these acts would be “momentary exceptions" in an overall upright life, it would be possible to justify grave faults in failing to live up to one's Christian life. An unfortunate consequence of this erroneous theory is that it undermines the person's capacity of decision and commitment in the use of one's free will.
Closely related to the previous theory is the proposal of a three-way division of sin into venial, grave, and mortal. The latter would represent a conscious and irrevocable decision to offend God, and would be the only sin that could separate us from God and close the doors to eternal life. Thus the majority of sins which, given their matter, have traditionally been considered as mortal would now only be “grave" sins, since they would not be committed with the positive intention to reject God.
The Church has frequently pointed out the errors that underlie these currents of thought. This is a theory that greatly weakens human freedom, since it overlooks the fact that in reality it is the person who decides, and who can choose to modify his deepest aspirations and goals through specific individual acts.  “It still remains true that the essential and decisive distinction is between sin which destroys charity and sin which does not kill the supernatural life: there is no middle way between life and death." 
2.4 Other divisions
a) We can distinguish between actual sin, which is the very act of sinning, and habitual sin, which is the stain left in the soul by actual sin, the debt of penalty and guilt and, if a mortal sin, the privation of grace.
b) Personal sin is distinguished in turn from original sin, with which all of us are born, and which we have contracted through Adam's disobedience. Original sin inheres in each of us, even though we have not personally committed it. It can be compared to an inherited illness, which is cured by Baptism (at least, by its implicit desire), although a certain weakness remains that inclines us to commit new personal sins. Personal sin, therefore, is committed , while original sin is contracted (cf. Topic no 7: “Elevation to the Supernatural Order and Original Sin") .
c) External personal sins are those that are committed with an action that can be observed from outside (murder, robbery, defamation, etc.). Internal sins, on the other hand, remain in one's interior world, that is, in our will, without being manifested in external actions (interior anger, envy, avarice , etc.). All sin, whether external or internal, has its origin in an internal act of the will: this is the moral act, properly speaking. Purely internal acts can be sins and even grave ones.
d) We can speak of carnal or spiritual sins, according to whether they tend in a disordered way to a sensible good (or a reality that is presented under the appearance of good, for example, lust) or spiritual (pride). In themselves, the second group is graver; nevertheless, carnal sins as a general rule are more vehement, precisely because the object that attracts (a sensible reality) is more immediate.
e) Sins of commission and omission : all sin involves a disordered voluntary act. If this leads to an action, it is called a sin of commission ; if, on the contrary, the voluntary act involves omitting some due action, it is called one of omission .
3. Proliferation of sin
“Sin creates a proclivity to sin; it engenders vice by repetition of the same acts. This results in perverse inclinations which cloud conscience and corrupt the concrete judgment of good and evil. Thus sin tends to reproduce itself and reinforce itself, but it cannot destroy the moral sense at its root" (Catechism of the Catholic Church , 1865).
Capital sins are personal sins that especially incline one to further sins, since they are the “head" of the other sins. These capital sins are pride (source of all sin ex parte aversionis ; cf. Sir 10: 12-13), avarice, (source of sin ex parte conversionis ), lust, anger, gluttony, envy, and sloth (cf. Catechism of the Catholic Church , 1866).
The loss of the sense of sin is the result of the voluntary clouding of the conscience that leads a person, out of pride, to deny that personal sins are what they are and even to deny that sin exists. 
At times we ourselves do not directly do evil, but collaborate in some way, with greater or lesser responsibility and moral guilt, in the sinful action of other persons. “Sin is a personal act. Moreover, we have a responsibility for the sins committed by others when we cooperate in them : by participating directly and voluntarily in them; by ordering, advising, praising, or approving them; by not disclosing or not hindering them when we have an obligation to do so; by protecting evil-doers" ( Catechism of the Catholic Church , 1868).
Personal sins also give rise to social situations opposed to God's goodness, which are called “structures of sin."  These are the expression and effect of the sins of each person (cf. Catechism of the Catholic Church , 1869). 
In the context of the causes of sin, we need to speak of temptation, which is the incitement to evil. “The root of all sins lies in man's heart" (Catechism of the Catholic Church, 1873), and our heart can be attracted by the presence of apparent goods. The attraction of a temptation can never be so strong that it oblige s us to sin. No temptation has overtaken you that is not common to man. God is faithful, and he will not let you be tempted beyond your strength, but with the temptation will also provide the way of escape, that you may be able to endure it ( 1 Cor 10:13). If not deliberately sought after, and if made use of as an opportunity for moral effort, temptations can have a positive value for Christian life.
The causes of temptation can be reduced to three (cf. 1 Jn 2:16):
The “ world ": not as God's creation, because as such it is good, but in so far as it spurs us, through the disorder of sin, to “turn to creatures," with a materialist and pagan environment. 
The devil : who urges us to sin, but does not have the power to make us sin. The devil's temptations are rejected with prayer. 
The “ flesh " or concupiscence : the disorder in the powers of our soul that stems from original sin (also called fomes peccati , tinder for sin). These temptations are overcome with mortification and penance, and with the decision not to dialogue with them and to be sincere in spiritual direction, without covering them up with “unreasonable reasons." 
When facing temptation, we need to struggle to avoid consenting with our will to the involuntary representation of the evil contained in the suggestion.
To combat temptations we need to be very sincere with God, with ourselves, and in spiritual direction. Otherwise we run the risk of deforming our conscience. Sincerity is a great means for not falling into sin and for attaining true humility. God the Father comes out to meet us when we acknowledge we have sinned, revealing the pride that tries to cover over our sin.
We also need to flee from the occasions of sin , that is, from circumstances that arise with greater or less voluntariness and that represent a temptation for us. We must always avoid occasions of sin when we are free to do so. When it is a matter of proximate occasions (with a serious danger of falling into temptation) that are also necessary (that we cannot avoid), we should do everything possible to turn them from proximate occasions into remote ones. Insofar as possible, we also need to avoid continuous remote occasions, which undermine the spiritual life and predispose one to grave sin.
Pau Agulles Simó
Catechism of the Catholic Church , 1846-1876.
John Paul II, Apost. Exhort Reconciliatio et paenitentia , December 2, 1984, 14-18.
John Paul II, Enc. Veritatis splendor , August 6, 1993, 65-70.
St. Josemaría, Homily Interior Struggle , in Christ is Passing By , 73-82.
 St. Augustine, Contra Faustum manichoeum , 22, 27. Cf. Catechism of the Catholic Church , 1849.
 Classically, sin has been defined as a voluntary disobedience to the law of God; if it were not voluntary, it would not be a sin, since there would be no true human act.
 St. Augustine, De civitate Dei , 14, 28.
 Cf..John Paul II, Apost..Exhort. Reconciliatio et paenitentia , December 2, 1984, 17.
 Ibid . Cf. Catechism of the Catholic Church, 1857-1860.
 John Paul II, Reconciliatio et paenitentia, December 2, 1984, 17.
 One commits a mortal sin “when a person knowingly and willingly, for whatever reason, chooses something gravely disordered. In fact, such a choice already includes contempt for the divine law, a rejection of God's love for humanity and the whole of creation; the person turns away from God and loses charity" ( Ibid .).
 Cf. John Paul II, Enc. Veritatis splendor, August 6, 1993, 70.
 Besides considering the act in itself, it is important to keep in mind that the judgment of persons should be entrusted only to the justice and mercy of God (cf. Catechism of the Catholic Church , 1861).
 Only a person who has a truly grave reason to do so and finds it impossible to go to confession, can celebrate the sacraments and receive Holy Communion, after making an act of perfect contrition, which includes the resolution of going to confession as soon as possible (cf. Catechism of the Catholic Church , 1452 and 1457).
 Cf. St. Josemaría, Friends of God , 243; Furrow , 139.
 Cf. St. Josemaría, The Way , 325-331.
 Cf. John Paul II, Enc. Veritatis splendor , August 6, 1993, 65-70.
 Cf. Ibid ., 69.
 Cf. John Paul II, Reconciliatio et paenitentia , December 2, 1984, 17; Veritatis splendor , 70.
 Ibid ., 17.
 Cf. Ibid ., 18.
 Cf. John Paul II, Enc. Sollicitudo rei socialis , December 30, 1987, 36 ff.
 Cf. John Paul II, Reconciliatio et paenitentia , 16.
 Combatting these temptations requires going against the current with fortitude, as often as necessary, instead of being drawn along by worldly customs (cf. St. Josemaría, The Way , 376).
 For example, the prayer to St. Michael the Archangel, conqueror of Satan (cf. Rev 12:7 and 20:2). The Church has also always recommended some sacramentals, such as holy water, to combat the devil's temptations. “From nothing do evil spirits flee more, never to return, than from holy water," says St. Teresa of Avila (quoted in St. Josemaría, The Way , 572).
 Cf. St. Josemaría, The Way , 134 and 727.