DV Magazine has two articles in its Technical Difficulties column, one last month (not available online for some reason) and one this month (available here, but only if you log in) that describe the benefits and pitfalls of chroma sampling.

Chroma Sampling is what is used by the various DV formats and MPEG video formats to reduce the amount of bits that are needed when compressing video. If you've ever seen 4:4:4, 4:2:2, 4:1:1, or 4:2:0, these will talk about what these numbers actually mean.

In summary, the numbers refer to the number of samples of luma (Y'), and the two components of chroma (Cr and Cb) that are contained in a 4 pixel sample area. Full resolution sampling is 4:4:4, whereby brightness (luma) and color components (chroma) are all sampled at each pixel. This gets you the most accurate representation. However, since the eye is much more sensitive to the changes in brightness than the changes in color, it is common to reduce the chroma signals in order to increase the compression of the signal.

4:2:2, for example, has 2 chroma signals per 4 pixels, 4:1:1 has 1 each per 4 pixels. What, pray tell, is 4:2:0? It can't possibly be that you don't sample Cb at all. Correct, 4:2:0 is actually 4:2:0 on one line and 4:0:2 on the next line.

The articles, especially the one that is available online go into some detail on how 4:2:0 can be made to work and where it falls short. Pretty interesting stuff.