In regards to Starcraft II, what is a "vod"?
VOD is Video On Demand. On this context, it means a video that is taken from the screen of an observer or from the players themselves (those are called FPVOD-First Person VOD). The video is taken by softwares like FRAPS, in most of the cases. They usually have audio commentary from "experts" in the game, but this is not required.
VODs are used usually when the replay is not available, i.e: in Korean professional league games.