What is the difference between x264 and H.264?

Find out for yourself - read all about it:

http://en.wikipedia.org/wiki/X264

http://en.wikipedia.org/wiki/H264

-DH

h.264 is a codec, and x264 is an encoder/encoding library to encode h.264 videos. It is a superior encoder to the quicktime h.264 encoder.

Yes, quicktime should play x264 encoded files fine.