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

Perhaps if you go back in time everything will be deterministic until the time you went back in time. If your grandfather is still alive and you go back in time to kill him try as you might you will never succeed as it never happenned. If you do kill your grandfather it will turn out to be his long lost twin or something.