.Video Codec.
ffdshow -
http://sourceforge.net/projects/ffdshow/
ffdshow is an open-source video codec for decoding/encoding various video and audio formats such as DivX and XviD, both of which are the most popular *.avi movie formats. Many movie files I'll share with you require this codec to run.
OGM guide -
http://ld-anime.faireal.net/guide/ogm-en
If you're really into anime, you'll eventually bump into OGM files. Here's a good guide on playing OGM files.