I wouldnt mind a helpful explanation. No lectures or sermons please. When a sinner is assigned to an area of hell he or she can't leave that area. They are judged by the degree of their sin and are doomed to suffer the consequences for all eternity. For example, Poalo and Francesca have commited the sin of adultery and are cast into the sceond circle of hell by Minos where they will remain forever.
Purgatory is somewhat different.
The souls here have not commited mortal sin and so have escaped hell. However, they are not yet worthy enough to enter heaven. They are guilty of lesser sins and cannot enter heaven until they have cleansed themselves. Purgatory is shaped like a mountain and souls slowly move up the mountain towards heaven. The levels of the mountain are set according to the seven deadly sins and each soul must be pass through each level doing penance in order to enter heaven.