do you want the common answer or the technical one? The common answer is it's an incompatible TV system, PAL being the better one, used in most of the world, and NTSC used in America, Japan and one or two other places (Theres a third, called SECAM, but only France and possibly Russia use that one) The technical answer is it has to do with how the colours are displayed and how many lines (i.e vertical resolution) the screen has.
I'm sure that with the new laws coming next year that remove the analog TV broadcasts in the US, it's just one step closer to the abolishment of the NTSC system. As it is now with Digital media, the main difference between PAL and NTSC, which was the way the color information was stored in the analog signal, is pretty much gone. This leaves the only real difference between the two being the number of Horizontal Lines on the picture, and the FPS. NTSC has fewer lines but a slightly higher FPS, where as the PAL system has more lines, but a slightly lower FPS. Now, once Analog is gone completely, there will be no real reason to keep the systems seperate other than forcing incompatibility across the world. And I'm sure there are some companies that will want to force the differences to remain in order to try and protect their stake in certain markets.
PAL will not be abolished while Cathode Ray televisions are still in existance. This is because it also links in with how the image is displayed on the screen. PAL stands for Phase Alternating Line, here's why. AC mains differs from DC in that it alternates between a positive and a negative voltage at a set rate, known as the frequency, creating a sine wave on an oscilliscope. From a peak to the next peak, or a trough to the next trough on the sine wave is a single 'wavelength' and the frequency is the number of these wavelengths passing a set point in one second, measured in Hz. In the UK, mains frequency is 50Hz, which means there are 50 wavelengths every second. Moving onto the television display, you have 576 horizontal lines and each of these lines need to be drawn. The way PAL works is it draws half of the lines on a positive voltage, and the other half on the negative, hence the name Phase Alternating Line, because the lines drawn alternate with the phase of the mains voltage. This also means that it takes two complete waves (a positive wave and a negative wave) to draw the full frame, therefore the PAL framerate is half the mains frequency, making it 25 frames per second. The US framerate is coincidentally half the US mains frequency, there is no technical reason why it is.
Actually there is a technical reason for it. In the US NTSC system, one screen is drawn in two fields. It skips a horizontal line each time, then goes back and fills it in. Don't ask me why they chose to do this. When the television signal was being developed they did a lot of odd things. So it takes two full scans to create one full frame. So isntead of doing a full 60 frames per second (since they use the 60Hz), they only do ~30 frames per second (actually a bit lower for other reasons). 60 Fields per Second, 2 Fields per Frame, 30 Frames per Second.
It's done in NTSC too, it is why the FPS is half the Line (or Mains for the Europeans) Frequency. Edit: I'll break out my old college books from my communications courses if you don't believe me, but it will take me a little bit, I don't know where I stored them. The "Phase Alternating Line" doesn't refer to it skipping every other line, this is a common feature between both PAL and NTSC. It instead refers to the way the color information is stored line to line. It is inverted every other line (which is the real meaning of "Phase Alternating Line"). Where as in NTSC, the color information remains in the same phase the whole time. Analog TV is a very messy monster. Especially NTSC, since PAL evolved from it they were able to correct some problems that were apparent with the NTSC color scheme. I'd prefer them to go all digital, but I doubt that'll happen in my life time, (And I'm only 25) look how long it's taken the US to get rid of analog air broadcasts (Feb '09).
