PAL
Some countries operate under a broadcast format known as "PAL" (Phase Alternating Line). PAL is used in countries in Europe, Asia and Australia.
Explanation of PAL
Images on PAL televisions are displayed at 25 frames per second, because of the 50 hertz power supply that is generated in PAL countries. These broadcasts contains 625 lines of resolution per frame.
NTSC
Rather than PAL, the United States makes use of a broadcast format called "NTSC" (National Television Standards Committee).
Explanation of NTSC
Since the United States generates power at 60 hertz, NTSC televisions display images at 30 frames per second. Despite this advantage in frames, the resolution of the images is a mere 525 lines.
PAL vs. NTSC for Wii
In the end, the differences in the PAL and NTSC images displayed by the Wii are negligible. One is no better than the other, despite the fact that attempting to convert a PAL image to NTSC--or vise versa--causes image distortion.