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

i thought BttF1 and BttF2 contradicted themselves somewhat
in part 1 he was starting to fade b/c his parents weren't going to be together.
in part 2 his interfering in the past created another timeline

so which is it? only one or multiple timelines can be created from interfering in the past?