Confession is about forgiveness. I know, I know, sort of duh. But that's what the whole point is. Forgiveness and (re)admittance into the Kingdom of Heaven.
We start out by saying that forgiveness is sort of antithetical to this world. We're taught from the beginning that nothing is free. If someone is offering you something, 'no strings attached', there're *hidden* strings, and we better be even *more* suspicious.
But that's what forgiveness is. Totally free, we just have to ask for it. And, even more, it's not just the 'slate being wiped clean', but the guilt is removed as well. "In confession, the forgiveness we receive is nothing less than an act of love. If not only wipes out the badness of what is forgiven, but also (to use the expression in the form of the Lord's Prayer we use in the Church) forgives what is 'owed'." The Mystery of Forgiveness allows the individual to start over with a clean slate - it allows us to get rid of all the trash in our minds, the thoughts and feelings that drag us down.
Confession requires a 'safe space' - a place that the person feels secure in terms of secrecy and confidentiality. A lot of what is confessed are things that shame the penitent. By definition, they are things that are hard to talk about, and they're certainly not things that we want everybody and their brother to know about. The penitent also needs to have confidence in the priest who leads them toward the Mystery of Confession. They must feel absolutely certain that what they say will be confidential, and that he will not react in a shocked or abrupt manner. The priest is not there to *judge* the penitent, but rather to be a witness. God is the judge, and God desires the repentance, the inner transformation, of the one who has gone astray. Everything revealed must be met by the priest with empathy and his presence. What I mean is that his job is to listen. To be utterly present in the moment, and *listen* to what the person is saying. Not think five moves ahead to what he's going to advise them. Not wonder what his wife is making for supper, or wondering when he'd bruised his shin. He must be *present in the moment*, which is something everyone needs to strive for, remember?
"Except in very exceptional circumstances, which will be described to you should they ever occur, the priest is not allowed to act on any of the information you tell him in confession. If there is some reason he wants to talk about something you mentioned in confession at a later date, he is supposed to ask your permission first, and if you say 'no', that is the end of the matter.
"There is an old story I have heard in different forms in different languages. A priest is tested by God when someone goes to confession and says that he has had copies made of the keys to a particular house. Later the priest discovers that it is, in fact, his own house that is being described. The end of the story varies, but it is basically aimed at demonstrating that a good priest would not then go home and have his locks changed. He behaves as if the communication during confession never existed."
Again, the priest does not stand in judgment, that's God's 'job'. He is there as a witness. It is important to remember that in confession, there are two sinners standing there - the penitent and the priest. The importance of the Mystery of Confession lies, ultimately, in the difference that it makes to the life of the penitent. "The Mystery of Confession starts with estrangement and alienation from God, and ends with the path toward theosis."
In regards to the content of Confession: spontaneous thoughts are not sins. Nor are spontaneous feelings, and temptations are not sins. *Acting* on our feelings may be a sin - *acting* on a thought may be a sin, depending on the feeling or thought. Giving in to a temptation may be a sin. One must have intent for their action for it to be a sin. Something that you do unconsciously, without thought or intent, is not a sin. You aren't there to talk about other people, and what wrongs they may or may not have committed. You are there to speak of your shortfalls and sins. No one else.
The purpose of confession is to tell God (through the priest), of the sins we have committed. Not because He doesn't already know, better even than ourselves, but because we don't want to, but we need to. People always want others to think that they're nearly perfect, creating this outer version of themselves. On the other hand, we also tend to think less of ourselves than we really are. Our inner view of our self doesn't match the truth, or our projection of ourselves to others. Confession helps to remind us that we're not perfect, but also to lift us up, to prove to ourselves that we're not as bad as we think of ourselves.
Lastly, sin and responsibility. We've already mentioned that a sin is not a random thought or feeling, but what we do with them. Briefly thinking of stealing a thing doesn't become a sin until you act on that thought. In the same vein, we are only responsible for ourselves. We cannot pin our happiness or goodness on someone else.