FPS in the video sense is actually Fields Per Second - there are two fields per frame. So your frames per second is actually half of your FPS. This applies to standard NTSC video. When you move into the territory of PC games and such things become quite a bit more convuluted. But if you ever see a game featured as running at 60 fps, but are confused as to how this can be when video only runs at about 30 frames per second, this is why.