.avi format:
The extension is .avi, but the coding can be different (DivX being the most common). I like .avi because my home DVD player plays most of these files so I can watch stuff on my normal TV, if not, my computer can always read them because I have download "codecs" from the internet and these are necessary for some .avi's. (My DVD player can't upgrade the codecs - but basically the cheaper the player the more Codec's it has (don't know why) and stay way away from Sony as they are crazy about intellectual property issues and basically cripple everything they make).
If you are having trouble playing .avi's on your computer, such as no sound, or a jumpy video, or it says it doesn't have the proper codec, try downloading Klite Codec pack (basic) and that should solve many problems. [top]

.3gp format:
This format is mostly for phones and portable devices (particularly apple). The audio is coded in aac rather than mp3, which is supposedly better, I have my doubts, but whatever. [top]

Palm Devices
If you are trying to run .3gp, or small .avi, on a Palm, the media playing program bundled is so totally useless the mind boggles.
I use the freeware program tcpmp, which plays many .avi's and .mp3's. There are licensing issues with it playing .3gp for some reason, so I have bundled the .3gp required patch into the download. Load all the .prc's onto your palm, and all should be well. [top]

Small Avi
This format is mostly for phones and portable devices as the sound is still stereo mp3, but the screen size is 300X240 - so there isn't the jitteriness that comes with large avi files on a portable device.