Hows it going??

I am getting myself totally confused here regarding quicktime mp4!!!

If I embed a quicktime mp4 file into a web page, will the Windows Media Player for the PC be able to play this file - or is it just a quicktime file?

How commomly used is mp4 and is it cross platform compatable?

Sorry if these are stupid questions, I think I have read too much information and my brain can't process it all - time to upgrade