This should have been the first blog post, as it’s about the most basic question.
Probably the most basic answer is that it’s the Sacrament of forgiveness and healing, wherein God himself forgives us of our sins.
But what is a Sacrament anyway? It is a set of visible signs that give grace — by the power of God, of course.
In the case of Confession, the signs (otherwise known as the “matter” and “form” of the Sacrament — but those are already technical stuff we don’t want to hear about for now) are (1) the “acts of the penitent” — that is, his being sorry for his sins (contrition), his telling of those sins to the priest (Confession proper), and his carrying out of the penance given (satisfaction); and (2) the granting of forgiveness by the priest who is acting in the name and in the person of Christ (this act is also called “absolution”).
Now grace is the invisible thing that our souls receive when we go to Confession. When we commit a mortal or grave sin, we lose the so-called “sanctifying grace.” But it is given back to us in Confession. Sanctifying grace is what we first received in Baptism; it is our sharing in the life of God — in other words, when we have sanctifying grace, God himself is living in our souls! Mind boggling, I know!
And whether or not our Confession included mortal sins, we also receive “actual graces.” These are ‘custom-made’ graces that can help ease our struggle to live as true Christians. These may be strength in fighting temptations, or some circumstances or events that can help us become holy (say, your crush smiles at you so you become more cheerful, or your prof gives you a higher grade so you can be encouraged to do better, that sort of stuff).
Actually, we can’t tackle all the subtopics under the definition of Confession in just one blog post. Unless you can endure a novel-esque post — which I’m sure you can’t, because neither can I and the rest of the attention-deficient online world.
So watch out for sequels. To give you a hint on how to look at Confession when trying to define it: imagine a finely cut diamond — it has many facets, and whichever facet you focus on, the whole thing still looks absolutely amazing. It’s the same with Confession: different sides, same beauty.