Friction in space and on Earth

This post is in response to a comment I got on my previous post "More thoughts on the importance of science in science fiction" where Shannon commented/asked (I'm only quoting the question-y part of her comment):

It really is a hard concept to grasp, the no-friction-in-space thing. I don't think I really get it - I'm not sure how to visualise it, for a start - but I don't understand how a space ship - of the super-advanced, sci-fi kind - can't slow down. I mean, it's mechanical and computerise...