distance between stars is calculated in light years which is the amount of time light travels between two stars. so eg if the distance between our sun and another star is one light year it will take one year using speed of light for us to reach to the other star. (do correct me if I'm wrong)

really love science fiction: the time machine (about time travel) and the most recent one, pandorum (space travel w/o light speed)

should read The Forever War too. I haven't read that myself, but it is about a war between two star systems. and the soldiers have to use faster than light travel. so centuries pass as they travel between the stars.