Mortal sins are the only type of proper, actual sin. This is reflected in the name, “mortal”, meaning a sin that can affect death (in a final sense, as well, the second death of Hell).
They are a transgression of the laws of God, and they break the sinner’s communion with God until the sinner repents and is absolved (either by perfect contrition, sacramental confession, or possibly other ways that the Church does not pronounce on, for Christians who do not practice confession). When a mortal sin is committed, the soul is deprived of justifying grace, and it must undergo the “third justification” (a return to a state of grace after a post-baptismal sin), which is effected by the three ways mentioned above. It is generally accepted today that a sinner can undergo the third justification many times. (In the early Church, repentance was public, very long [years], and generally only allowed once after baptism. Thus some of the early Christians delayed baptism to varying degrees, into adolescence or later.)
People who will be in a state of grace at their death are called by God those who have been given the grace of perseverance: they will achieve heaven, and are larger in number than the canonized saints. (This doesn’t mean immediate access to heaven; not all people who die in a state of grace bypass purgatory, although, once in purgatory, heaven is guaranteed: there is no way from purgatory to hell, contrary to an opinion I find amongst some people in the pews that purgatory is a “sorting place” for souls.) In other words, only mortal sins can keep you out of heaven, by depriving you of justifying grace, so, while, in mortal sin, one stands before God as unrighteous, instead of standing before God clothed in the Iustitia Dei through Christ, and of Christ, and in Christ, as the sinner does when he is justified (or, as Bultmann put it in one of his moments of brilliance, “rightwised”) by and before God.
Contrary to popular opinion, the “seven deadly sins” are not sins. They are habitual inclinations to sin, but not themselves sinful. Say, someone who is affected by the “deadly sin” of lust, is not thereby a sinner: but the lust predisposes him to an actual sin in adultery, for example.
By a habit I mean not a created supernatural thing in the soul (the “habit of grace”), nor the deprivation of it (habitual inclinations to sin), which is the context the concept arose within, the mediaeval theological opinion of “created habits of grace” (which I reject, as did St Thomas). I speak of “habits” in either sense as only a psychological disposition gained by performing or avoiding the performance of a specific action, not of “habits” which are a basis for the acceptatio divina.
Other so-called “sins”, the venial sins, affect your relationship with God in no way. They are more like the “seven deadly sins”, in that venial sins (which are not sins properly so-called), are merely habits that make it more likely for a man to actually sin; they desensitize and inure him to sin; they eventually provide near occasions to sin: but they themselves are not sin. (For example, “idle talk” is considered in all older and some newer manuals a venial sin: it is not itself sinful, but can lead to sins of blasphemy, bearing false witness, etc.) Habitual performance of venial sin may harm your relationship with God, but even myriad venial sins do not destroy it.
Only mortal sin is “sin properly so-called”, that being, an action that destroys an individual’s relationship with God. Other things called “sin” or “near occasion to sin” generally are habits, vices, etc. that desensitize, habituate, and inure an individual to sin, making it more likely for him to commit an actual (mortal) sin, and, since accustomed to the sin, to make it harder for him to see the scandal in it, or the wrongness of it, and thus make it harder to repent.
“Harmatology” - the study of sin - has had thousands of books written on it; we can not define it in depth here, although I refer you to the appropriate sections in the Denzinger (look it up in the analytical index), The Fundamentals of Catholic Dogma (again, index), the Catechism of the Catholic Church (§§1846-76), and the Roman Catechism (or the Catechism of the Council of Trent), in the chapter dealing with “one baptism for the remission of sins”, which is the rubric under which it deals with harmatology.
Please note that my writing above is theologically inexact when it comes to the decrees of election, predestination, efficacious grace, etc.; I write from the human perspective of “what?”, not the Godly perspectives of “why?” and “how?”. As such, if taken literally, my above post could be spoken of as having Pelagian tendencies, because I dwell little on grace in it. I am not a Pelagian, but in fact a strict Bañezian-Thomist (i.e. “Catholic Calvinist”), but have no desire to turn my answer to the question in to a treatise on soteriology and the grace of God.
I will try to crack it open for you in simple terms.