The first form is the one you are probably used to. Individual, you go on a Saturday (or other day) into the confessional and celebrate the sacrament.
Form two is a communal penance service followed by individual confessions. There is usually some form of Liturgy of the Word, maybe an examination of conscience. Sometimes there is a communal act of contrition or other expression of sorrow. There could be singing or even a brief homily. Then there are individual confessions, which is necessary if a person has a mortal sin. Often a parish does these during Lent and Advent when a lot of people want to go to confession. They sometimes bring in extra priests.
The third form is very rare and that is general absolution. That is usually done in very extraordinary circumstances, usually life or death. It is when a priest absolves a large group of people without going to confession. For example on a sinking ship or for service men and women about to go into battle. The thing is, if you have a mortal sin you are still expected to go to confession at the first opportunity if you have a mortal sin. For example if you are rescued off the sinking ship when you get to port you should go to confession soon. If you die on the ship you don’t have to worry coz your sin is forgiven. That is sort of an easy way to describe it, it is a bit more detailed than that and as I said, it is very rare and the Vatican sort of frowns upon it. There have been priests who were doing it all the time, even when people could get to confession.
