Codex

[x9protocol]

I understand that they let you watch a movie that you would not be able to watch without them but what is all the different codex about? Explain what they are exactly and what is the codex for ac3?

[wolleric]

A codec (COde/DECode) is a software layer that is used by any kind of video/audio software (player, converter etc.) to get the raw pixel/sound data from an encoded file. Take for example an MP3 file. It doesn't contain pure sound data that can be sent directly to the sound card. It has to be decoded first (that's why playing an MP3 file uses much more CPU time than playing an uncompressed WAV file).
Maybe MP3 is not the best example since MP3 support (the "codec") is included in every native MP3 software player. However, an AV editing tool like VirtualDub won't play MP3 audio without the correct codec.
AC3 is another audio encoding sceme. It can contain up to 6 channels and is the standard on DVDs. There is a codec that allows you to play AVI movies with AC3 sound in Windows Media Player. Maybe there is a software that can play AC3 files directly, I haven't found any. Or you use BSPlayer, it has integrated AC3 support and doesn't need a codec.

[KingJohn]

I understand that they let you watch a movie that you would not be able to watch without them but what is all the different codex about? Explain what they are exactly and what is the codex for ac3?

This may help
http://www.vcdhelp.com/forum/userguides/135217.php