Hell
Ruby: Actually, they got it pretty right, except for all the custom leather.
3.09 Malleus Maleficarum.Hell in Supernatural, is the place or punishment and pain of the afterlife. Human souls who are sent to Hell are tormented until their humanity is stripped away and they become demons (3.09 Malleus Maleficarum).
Humans who sell their souls to [{Crosroads Demons]] or other demons go to Hell. It is unclear how or if other humans end up their.
According to Alastair, John Winchester spent the equivalent of 100 years there. He escaped through the Devil's Gate in Wyoming during the events of 2.22 All Hell Breaks Loose: Part Two
Dean spends four months in Earth time in hell, but he reports that time passes differently there, and for him it was more like forty years (4.10 Heaven And Hell). This makes 3 days on earth roughly equivalent to a year in hell.
During his time in Hell, Dean was tortured and each 'day' his wounds would be healed, and Alastair would offer to stop it, if Dean would become a torturer of souls himself. After '30 years' in hell (or 3 months on Earth) Dean accepted the offer and started torturing souls. This broke the first of The 66 Seals, starting the process to free Lucifer.
Alastair also made the same offer to John Winchester that he made for Dean, however took the pain and the torture until his escape.
Castiel reports that angels laid siege to Hell to rescue dean before he broke the first seal. Eventually it was Castiel who rescued him.
Some demons appear to be able to move freely between Hell and the mortal realm eg Azazel, Alastair. Others are trapped and can only escape through portals such as Devil's gates.
When a demon is exorcised, it returns to hell whereas the Colt, Ruby's Knife and Sam kills, as in obliterates, the demon.