Well, I've studied The Theory Time Travel for months now. Aside from Quantum Physics and Space, Time Travel is relatively dealing with basics of surfing through the +Fourth Dimension. Basically, the Fourth Dimension (or more) is another dimension after the third, and brings the theory of Parallel Universe Traveling (or alternate reality). If there is Parallel Universes, then there is many differences happening but we do not live in it and no way possible except time traveling to it.
Imagine you walk to Wawa this morning to grab a cup of coffee, what if there was a different way happening at the same morning where you didn't get your coffee? That's Parallel Universes going on that we do not coexist with.
One's who believe in Time Travel obviously have to believe in the fact that surfing through parallel dimensions is just as equivalent if not more. Another word for this is channels, like your TV. You go to 66- Cartoon Network, but many other channels are going on at the same time, But since I'm busy watching Family Guy I'm going to miss living to watch the football game live, first time.
There has been two conventions known to history about discussing time travel, one was in the 80's and one recently in 2003- many came and anticipated to see people from the future. Unfortunately, nobody from the future came; many believe that it was all fake, many others believe they flew themselves into parallel universes.
Now the other theory of time travel, to make things sound even stranger, is that one who time travels obviously can't alter the history and practically becomes a ghost from everybody else- paranormal activity that most will think are ghosts.
NOTE IMPORTANTLY: That I do not know if I am right or wrong with any of the information. I didn't want to go knee deep with time travel theoretical information, but I thought this will be enough information to satisfy you.
For Fun: If you like dramatic movies with time travel, watch Donnie Darko, very good movie.
I'm a 16 year old studying theoretical physics. One part of which helps greatly with your answer. I hoped this all helped and I enjoyed answering your curiosity : )