Qui or ce qui?

Qui and ce qui can both be translated as "which":
Le film qui est sorti la semaine passé a été très populaire. - the film which came out last week was very popular
Beaucoup de gens sont allés le voir, ce qui ne m'étonne pas. - A lot of people went to see it, which doesn't surprise me
Qui refers specifically to one thing or person.
Ce qui does not refer to a specific noun, but rather to an idea or an entire statement, which may or may not have been previously expressed.

You can find more examples of the use of qui and ce qui on the excellent Tex's French Grammar at http://www.laits.utexas.edu/tex/index.html There are even audio files so that you can hear the examples given, and interactive exercises to test your knowledge.