Hi , I am a complete newbie (moron)

I have downloaded many movies and the only ones that have opened and worked are ones that have an ending (.mov)
everything else won't open
I get a message from quicktime saying 'quicktime has quit, bla bla bla
What is going on and is there something I need to download to get the movies to play?
I am os x
please help in plain language I can understand
Thanks

Cazza