CODECs are all propeitary video compression & decompression algorithms that are developed by independent companies or consortium of companies, so they may not playback across all media players and devices unless they are licensed to do so.
Eg. under a Quicktime Movie wrapper, the codecs used can be Motion-Jpeg compression, the whole family of MPEG compression, H264 or even uncompressed video, etc...
Same theory applies to .AVI formats.....the encoded video formats can be in Sorenson, MS RLE, or any other compression formats compatible with the .AVI wrapper
MPEG compression formats like .MPG, .MTS, .MP4, etc...are more common across most media players.
However, when it come to .wmv, .flv, or other video formats that are more likely to be propeitary to a certain company, then these formats may not be able to play with your common video media players.
Hope this helps a bit.
