NTSC = Never The Same Colour, I mean, National Television Standards Committee. NTSC is the video system used in the USA, Canada, Japan, and certain other countries (usually in proximity to the USA, or (former) territories of the USA).
PAL = Phase Alternating Line, used just about everywhere else except for France (sigh) and some former French territories which use SECAM. SECAM is infrequently encountered and I won't talk much about it.
Anyway, the difference between PAL and NTSC is in the screen size, how frequently the screen is redrawn, and how the colour signals are created. TV screens are interlaced (half the screen is drawn at a time, first even lines, then odd lines). NTSC has 525 screen lines, and it draws a half-screen every 1/60th of a second. PAL has 625 screen lines, and it draws a half-screen every 1/50th of a second. (Ironically, even though PAL has more lines on the screen, if you compute out how many actual "pixels" (so to speak) it updates over time based on the ITU standards, NTSC is actually higher resolution.)
Additionally, they also display colour differently. PAL makes up for its 50Hz flicker by having excellent colour stability and actually correcting the colour signal on alternate lines (done by reversing the signal phase, hence Phase Alternating Line). NTSC tends to have poorer colour quality and a greater tendency for spectrum drift, which is why it gets called Never The Same Colour since one colour one place on the screen is frequently not the same colour elsewhere, even if it's supposed to be.
Anyway, all this goes to say that NTSC and PAL are dissimilar standards. If you pipe a PAL signal into an NTSC television, besides some picture roll, you'll get a black-and-white picture (no colour since they encode it differently), and vice versa.
One thing that is nice about NTSC is that NTSC is NTSC, no matter where you go. An NTSC American TV will pick up Japanese NTSC channels, and vice versa. Thus, there is no reason other than pure greed why a Japanese DVD can't be played on an American DVD player.
PAL, on the other hand, is a big mess. There are multiple kinds of PAL signals, and most of them are partially or completely incompatible with other varieties. The UK uses PAL I (I think), most of the rest of Europe uses PAL B (except, again, France), Eastern Europe uses something else (PAL K?), most of South America uses PAL N (except Brazil which uses PAL M), Australia uses PAL G, and Red China uses PAL D but Hong Kong uses PAL I too!
The silver lining is that most European and Asian television sets receive and understand multiple television standards, including all the PAL variants, often SECAM too, and usually NTSC to boot. Well worth the investment if you deal a lot in foreign movies (might as well get a foreign DVD player while you're there so you have the right region code).
By the way, HDTV is not going to help. In fact, it's already making things worse. Japan's HDTV is NOT the same as the American HDTV (so long NTSC), for example, but that's a topic for another day.
Sorry you asked?
NB: Note that PAL and NTSC DVDs mean the spatial resolution and refresh rate only -- DVDs use MPEG colour encoding, so the colour signal difference doesn't apply, but the refresh rate and line count will still ruin your day. This makes no difference to your computer, though, which just displays a different sized window, and updates it at the right rate.
