region 0 discs are uncoded and can be played worldwide...but you have to play PAL discs in a PAL-compatible player and NTSC discs in an NTSC-compatible player.
A/V information can be placed on DVDs in one of two resolutions; 720 x 576 pixels (PAL DVDs), or 720 x 480 pixels (NTSC DVDs), and with various frame rates (24, 25, and 30 frames per second are common). The DVD player itself takes this data file and formats it appropriately for display in either PAL or NTSC.