divX and avc is a video format? what are flv and mp4?

[Iskandar]

i did some readings, they say divx, avc and some others are video formats. "MP3 is to audio what DivX is to video". if those are video formats, then how about fla, avi, mpg, mkv and others? is there more than one category of video formats?
erm okay, so if those are all video formats, then why the extension is mkv then i open media info it says avc? is it the same?