flv is the flash video format youtube uses for it's videos along with many other video streaming sites. iPods can't read FLV files so you need to get a program to convert the FLV into something the iPod can read. I've messed around with a program called tubehunter, it does conversions automatically, except if you're just downloading music for some reason it won't convert the flv's into mp3 even when you tell it to. Oh well, it still works for videos.