There are actually 3 kinds of sin the Church recognizes. I’ll describe each of them below.
Original sin - this is NOT an actual sin. Rather, it is a state that a person is born into. It is the loss of sanctifying grace (God’s grace dwelling in us) that came from Adam and Eve. We are freed from original sin at Baptism, but the effects still remain in that we still sin. (I’d liken it to phantom pain after an amputation. The limb is gone, but the nerves are still there - so you can still get pain in the limb.)
Mortal sin - this is a serious sin. It’s characterized by grave matter (something that does significant harm to us or someone else, or is a serious offense against God), total knowledge of the evilness of the action, and complete consent of the will. Mortal sin destroys sanctifying grace. Someone who dies in a state of unrepentant mortal sin will go to hell. All mortal sins must be confessed, at least once a year, and a person cannot go to Holy Communion in a state of mortal sin. Confession restores sanctifying grace.
Venial sin - this is a less serious sin that involves less serious matter, incomplete knowledge, and/or incomplete consent. (Accidents, for the record, are not sins. So if someone accidentally smacked into someone walking down the street, that’s not a sin. A sin must be deliberate.) Venial sins do not destroy sanctifying grace, but they do weaken God’s grace. We’re not obliged to confess venial sins, but it is useful to do so. NO amount of venial sins can ever equal a mortal sin, but they can make us more likely to commit a mortal sin.
A few more notes about sin:
-Sins can only be committed by a person who has reached the age of reason. This is defined as age 7.
-Baptism, which can only be administered once, removes original sin and gives a person sanctifying grace. If the person has reached the age of reason, it also removes any personal sins.
-Venial sins can be forgiven during the penitential rite at Mass.
-Those who die with venial sins on their souls go to Purgatory after death. (This is referenced in one of the books of Maccabees.) In Purgatory, souls who are going to heaven are purged from any temporal punishment remaining. (Christ came to abolish eternal punishment, not temporal punishment.) These people are NOT waiting judgement; they have already been judged. They’re going to heaven, but it will take some time for them to get there, because nothing unclean can enter heaven (it says this in Scripture). We can pray for the souls in Purgatory, and it is an efficacious thing to do, especially to have Masses said for them and to pray the Way of the Cross for them. (This is a common source of confusion for Protestants, because many cite the “selling of indulgences”. Sadly, it is true that some priests engaged in this practice, which is the sin of simony - buying and selling spiritual things. Today, indulgences cannot be bought. It is customary, if one wishes to have a Mass said for a soul, to give a donation, and usually if one wishes to light a candle for a prayer intention one gives a donation to cover the cost of the candle. However, this is NOT “selling” indulgences.)