Each one teach one I say. TinkerNut's video is vidgeeky but does a concise job of explaining the various video formats, what is a codec and some of the software used to convert from one video format to another.
TinkerNut mentioned some good video conversion software products like Format Factory, Enrightsoft SuperC and Handbreak.
I would like to put a special shout out to Squared5's MPEG Streamclip I would always to go the source to download software unless it was from a trusted source like Major Geeks or CNet's Download.com.
You have to be more careful these days where you download stuff. And for goodness sake for the next few weeks use Opera or another secured browswer. Leave I.E. alone until they patch that rascal proper.