co-dec
compressor de-compressor.
raw video files are way too large to transport over the internet. you use a compressor to shrink the file to a internet friendly size and a de-compressor to un-shrink it and play it on your media player.
the most popular codec on the internet is Divx. you get it from divx-dot-com. after you install it your media player will automatically be able to play divx compressed movie files.