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).
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 equivelant 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. According to Alastair John spent some time in Hell, this was reveled in 4.16 On The Head Of A Pin. Alastair also made the same offer to John that he made for Dean, John took the pain and the torture for 100 years and then he finally gave into Alastair’s offer. 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.
- An angel is capable of snatching a human soul from The Pit. As of now that only known thing that can snatch a human soul from the depths of Hell is an Angel of God.
The Demonic Staff of Hell
Like Angels, Demons have some sort of a chain of Demonic command. This theory has been reveled in many Episodes, in which these episodes completely explained the following:
- Demons, like angels have some sort of a line of authority. The rank of a Demon determines where they fall in the demonic chain of authority.
Lucifer- is a fallen Archangel and former warrior of Heaven, even thought Lucifer isn't a "Demon", demons apparently have a belief system, just like ours and Gods plan; their belief is that Lucifer as their Creator is their God and father. Most of them hold Lucifer in High amounts of esteem and have much respect for him. One demon named Crowley, however doesn't seem to like him, in fact he ask the Winchesters to kill him with The Colt.
Lilith- was the First Demon that Lucifer created, as an act of defiance towards God.
Azazel- is, ironically the only Demon with yellow eyes; this separates him from normal demons. He is extremely powerful, and he is known for creating acts of chaos and mischief. In 1972, apparently, Azazel was the very first demon to discover the location of Lucifer's "cage door". Apparently Lucifer was imprisoned some-where deep with the the convent of St. Mary's, it was there that Lucifer gave Azazel his first mission, which was to find a "very special child", in other words, Sam.
White Eyed Demons:
Alastair- is the Grand Torturer of Hell, he is known for having tortured Dean and John Winchester. He is extremely powerful, according to Ruby. Alastair has displayed great amounts of Demonic strength and resistant’s; even after his First Meatsuit was destroyed by Anna's angelic blast when she retrieved her grace, apparently it didn't kill him, instead, his meatsuit was ultimately affected by the blast. 4.15 Death Takes A Holiday
- It must be noted that, Lilith is both a high-tier demon, as well as a white eyed demon.
Other Important Demons:
Ruby- is Lilith's Right-hand Woman, according to her, not even Alastair knew her true purpose, as it has been said many times, only Lilith knew her true mission.
Known Humans that went to Hell
- John Winchester (100 Hell years), Rescued by: An Unknown Angel
- Dean Winchester (30 Hell years), rescued by: Castiel