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PAL (Phase Alternation by Line) and NTSC (national television standards committee) are regional video standards. PAL is used in europe, and NTSC is used in North America and Japan. It defines how images are displayed on your screen.
NTSC has 525 lines of vertical resolution at 60hz, meaning the screen is drawn 60 times a second. NTSC is also refered to as "30 frames per second," because only half the screen is drawn with each pass, so the entire screen is redrawn only 30 times. Each of these passes are refered to as a "field," so 60 fields per second are drawn on the screen, each field containing half the image.
PAL has 625 vertical lines, 100 more than NTSC, and has a refresh of 50hz, resulting in a frame rate of 25 frames/sec.
So basically, NTSC and PAL represent differing standards of displaying video images, and differ in the resolution of the image, and the rate at which the image is displayed.
Video devices, such as dreamcasts or DVD players, have to be aware of and configured to output a video signal that matches the display device.
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