PAL (phase alternating line) is a colour encoding used in analogue televisions, and now on DVDs (and games) in reference to frame rate. PAL, as opposed to NTSC (National Television System(s) Committee), runs at 25 frames per second, while NTSC is 30 frames per second. If you convert PAL to NTSC then extra frames are added and eventually the aurdio and video go out of sync, where-as converting NTSC to PAL means that frames are dropped.
I suggest looking up both on wikipedia for a more complicated explanation.