Help for video formats
Since I'm a Windows only user, I can only offer assistance when it comes to Windows! I have no idea how to get Xvid to work on a Linux operating system!
Format: Description:

ASF: Active Streaming Format. Microsofts attempt at achieving a good video streaming format (and put Real Media out of business). Never really got any good quailty with it though. As long as you have Windows Media Player 6.4 or above, you're fine.

DivX: This one became popular for it's compression ability without losing too much quality, although that depends on the person coding it! Head over to DivX.com and download the latest codec. ALWAYS check for the latest codecs!

Flash: Macromedias web media streaming software. We usually avoid it since you can't really do anything with thise and they either require Macromedias own player or comes as an EXE file.

MPG/MPEG: This is an oldy and goldy that is compatible with most operating systems. Doesn't compress too much so the file size is pretty big. The two usual standards here is either VCD format or SVCD. Normal VCD doesn't require anything special, but SVCD can be tricky. You'll need an MPEG2 codec. They usually come integrated in every good software DVD player, like PowerDVD or WinDVD. And you should play the files through the DVD player, not Windows Media Player!

Quicktime: Apples very own standard that works pretty well for both normal files and for streaming. Only problem is that it's not only video but it's got a whole set of attributes that may prohibit using the file the way you want! Head over Apple.com.

Real Media: One of the oldest format of video streaming. Pretty good quality/compression ratio but a bit too much hassle downloading, installing and configuring! Head over to Real.com if you really, really want it.

WMV: Windows Media file. Don't need anything special to get this to work on a normal Windows Media Player.

XviD: This is a pretty new (about a year old) codec that offers very good compression and quality! You'll need to get the latest codec. I've found it easier to play XviD vids through the DivX player than the normal Windows Media Player!