Questions About Downloading Video

I am really confused. I hope someone can help me. I got LimeWire so that I could use it like Napster for movies. I want to be able to download movies like I do MP3s from Napster and play them (Is the technology not there yet?). There are so many formats though. MPEG, MPG, AVI, ASF. WHATS THE DIFFERENCE!?! I have 4 movie players, but I'm not satisfied with any of them. I have Windows Media Player, Real Player, Quicktime, and something called "The Playa." Most of the time none of these can read the video clips I download. What will become the standard for digital video? I have read about something new called divx, at www.divx.com. Lots of things seem to use this format, but I can't get any of my players to read this. I have downloaded something called a "codec" for OpenDivx, but I still can't read some videos. Apparently there are some backwards compatibility issues. Does anyone know where I can get the previous version of the Divx codec?

I am totally lost. Napster was sooooo much easier.