It's a coder-decoder.
A nice place to start is with terabit's AVI info program.
DivX digest (no, it's not a porn site) has an <a href="http://www.divx-digest.com/software/terabits_avi_info.html">info page</a> and a link tio the download.
After you find out which codec you need, you'll have a better chance at finding the right one.