Eternal TortureThis is probably the image that most people have. Fire and brimstone, demons with pitchforks, lava and torture devices.
Whatever the method, this is the Hell where sinners go to suffer painful punishment forever.
This flaming pit of horror is probably first in most minds because it's the one we see most often depicted in art and movies.
Eternal Separation From God
This is the view that probably most theologians who understand the Biblical description of Hell hold.
This Hell is not so much a punishment for disobeying God, more the outcome of freely choosing to live without God. Many Christians see the afterlife in Heaven or Hell as not a reward or sentence, more the result of a free choice to be with, or go against, God. If a person does not want to be with God, then because of his love, justice, and mercy, he will not force them to be.
Unfortunately for the person who does not want to be with God, the outlook is necessarily bleak. God is the source of goodness, light, love, hope, joy, justice, and mercy among other great things. So to live without him, is to give up all that stuff. So although someone who goes there might not be in any physical agony, they'll be tormented with a hopeless, empty existence.
To paraphrase C.S.Lewis: this Hell is a jail cell that has been locked from the inside.
AnnihilationThis third view is that Hell is not an actual place where sinners are sent. Instead, at the end of days, the sinners will be thrown into a fire where their bodies and souls will be destroyed completely. They will no longer exist at all.
I did say that I wasn't going to go into which view I held, but I will briefly state that it's this one. Besides the Biblical verses that I think make this one clear, I'd generally expand on the point made in the 'separation' section. As well as all those good things, God is also the source of life. So if a person is sent away from the source of life, surely they would die. Seems straight forward to me.
I also think that God's ultimate goal is to create a perfectly good world. I'm not sure a perfectly good world would have a fiery pit of torment in it. It makes more sense to me that God would annihilate any evil and make sure it didn't exist at all.