PAL and NTSC are television standards that are used in different parts of the world. United States is a NTSC country while most of Europe and Asia are in PAL. What does that really mean? Well if you bring a DVD or tape from overseas, chances are it will not play in the US. The reason is NTSC DVD players are not compatible with PAL discs. You will need to convert the footage from PAL to NTSC in order to view it. On a contrary, most PAL DVD players in Europe or Asia play NTSC discs, so if you need to send someone overseas a DVD there is usually no need to convert to PAL.
One important note is that all computer-based (PC and MAC) DVD players will play both PAL and NTSC discs. If you are QC'ing a disc on a computer, you won't know whether it is NTSC or PAL because it will play both! ALWAYS QC your work on both computer and set-top DVD player.