I've got 2 files (movie clips) that I want to watch, but they are avi files
when I click to open it I get this message 
The document “33.avi” could not be opened. A required codec isn't available.

so my questions are
1) what is a codec?
2) where can I get one?
3) can I download something else to make these files open?

any advice would be helpful thanks