"On the Diversity of Sins
There are some sins of weakness, others, of ignorance, others of malice. Weakness is the opposite of strength, ignorance is the opposite of wisdom, and malice is the opposite of goodness. Consequently whoever knows what is the strength and wisdom of God can judge which are the pardonable sins. And whoever knows what is the goodness of God can appreciate which sins are due some kind of punishment both in this world and in the one to come. Once these matters have been adequately treated, one can plausibly determine those who are not to be compelled to a penance full of sorrow and mourning, although they acknowledge their sins, and they for whom no salvation whatever is to be hoped except they offer in sacrifice to God a spirit broken through penance."
Here is written some of the pastoral advice from St. Augustine the priest and abbot, regarding the different kinds of sins and their severity with relation to God and the Church. The first kind regards a sin of weakness, which results from a lack of strength (or humility to ask for grace) to do what is right, and the second kind is a sin of ignorance, what seems to me what St. Augustine calls crimes later on, though it may not be a sin if we have no fault in our own selves being in ignorance. Those who know the strength, wisdom, and goodness of God can relate which sins are pardonable, and which sins are due both a punishment in this world and possibly a punishment in the next world. It strikes me that St. Augustine's grouping of strength and wisdom in the first category regarding sins that are pardonable (venial in Latin, that is pardonable, forgivable) in that these sorts of sins regard a lacking of will to strive for God's love and heart, while sins of malice are ones that completely miss the point of God's goodness and go completely against Him as opposed to a lack of will. The venial sins relate a pardonable offense which can be remedied through prayer (see St. Augustine's Sermon to the Catechumens) and other sins for which a public penance full of mourning and an understanding that this sort of sin excludes a person from Communion with the Church and hence from salvation. This is why St. Augustine says that under careful consideration it can be considered whether one has to not do public penances full of sorrow even though they recognize their sin (in the case of venial sins) or do public penances and mourn the loss of their salvation in friendship with God (mortal or grave sins).
Herein we see the Catholic teaching of mortal and venial sins though St. Augustine is careful here, I think to state the pastoral concerns indicated by the word plausibly. There are many Catholics who fail to understand that mortal sins are sins by which our own heart has been closed off to God and other sins which stem from our weakness and failure to follow God indefinitely. Mortal sins always involve weakness and failure, but also an additional malicious intent to love a created thing above and beyond loving God. The distinction can be difficult sometimes if one wants to push the vagueness of my response. It will have to wait for another time since it is so late however.