Time Travel

So I just got done Watching tonight's episode of Lost. No spoilers if you didn't watch, but I will say it involved time travel. Lots of it. The more I watched the more I kept yelling "No. NO. That doesn't make any fucking sense, time doesn't work like that, you braindead plebs" at my tv. I know time travel is mostly theoretical, but can /sci/ explain time travel to me as best as they can? I'm sure you understand it better than Lost's writers do

When people go back in time and try to change history, the outcome is already determined. After all, they already were there and back again before they even left.
Gah. Where is the quote am I paraphrasing here from? I just can't remember.

Anyway, time travel involves 4-dimensional space-time, so classical Newtonists need not apply.
You have the trousers of time, which should lay to rest any silly notions about bathrobes, or you have an internally consistent time loop, in which a reaction can be its own cause, that is, there doesn't have to be a reason as long as the 4-dimensionally structure is well-formed.

The time loop seems unlikely, since operating by the rules we know, large systems don't end up in the exact state they started in, e.g. your grandchild ending up with your exact genes is an biological improbability, and for him/her/you to also at the end of the loop consist of all the same atoms you consisted off, you'd basically have to eat yourself. It may be that the existance of a time loop would coerce the space-time structure to make the pieces fit, though.