Anyone interested in this sort of stuff should give "The Elegant Universe" and "The Fabric of the Cosmos" by Brian Greene a read... both awesome books that deal with a lot of complex physics, but in terms that you can understand
Fabric is probably easier and a little more general, so start with that
Basically everything in the universe travels through space-time at the speed of light... so therefore the faster you travel through space, the slower you have to travel through time
Anyway this page explains things better than I can
Clicky