Fer cryin' out loud...
Know the difference between an architecture, track, codec, and program. An architecture defines the structure of the file and how all the tracks and other data are organized within it. A track defines an aspect of the data (audio, video, sprite, etc.). Tracks for certain types of high-bitrate data (namely audio, video, and still images) have to be compressed using lossy or lossless compression techniques. This compressor-decompressor code is known as a codec. A program has all the necessary architecture parsing and codec code to make or read one or multiple kinds of files.
So Quicktime is both the name of an architecture, and of a program that can read Quicktime, AVI, and MPEG architectures, and has most of the necessary codecs. MP3 is a lossy audio codec available to Quicktime, AVI, and MPEG. Windows Media Player can read windows media files as well as AVIs. MP4v3 was a microsoft video codec based on a non-finalized version of Mpeg 4 and available only to the Windows Media file format, until it was hacked to allow it to be used in AVIs and called DivX, which became hugely popular and eventually was brought to Quicktime (sort of).
Quicktime can open AVIs. However there is a non-standard way of encoding avis where the data is padded with zeros at certain key points, without explicitly specifying the start point. Quicktime can't read these AVIs properly, and ends up dropping the Audio. On top of that there is the DivX problem. MS tried to squash it on the PC side but was never really successful, and various versions are readily available. But due to the extremely poor Windows Media Player implementations on the mac, and the AVI encoding problem, the mac solutions for viewing these very popular files were tricky and not well known. For a long time the only option was the DivX player (mac.divx.st). Nowadays people have finally reverse-engineered the codec and are producing kosher versions for OS X and classic including divosx (divx.jamby.net), as well as divx alternatives like 3ivX (
www.3ivx.com) and the confusingly named DivX package (
www.divx.com) which includes both an alternative codec and an old style divx codec. The AVI problem remains only partially resolved via DivX doctoring or the AVI2MOV utility until - it is rumored - Quicktime 6, at which point we will finally have a total solution.