DVXUser has a concise, easy to understand explanation of colour sampling methods in digital video, with diagrams!

You’ve seen the numbers: 4:2:0, 4:4:4, 3:1:1, 3:1.5:1.5, 4:1:1, 4:2:2… What does it all mean? And how does it affect your video? What’s better, what’s worse, and when does it matter?

What those numbers are referring to is a technique in digital video commonly called “color sampling.” The concept can be a bit confusing to those not used to working with video in the digital domain. In video, it’s common practice to not actually record all the color in an image, but rather to average the pixels together to cut down on the bandwidth. Color sampling is, in effect, a form of compression. The more compressed the engineers can make the color channel, the less bandwidth the signal occupies and the easier it is to record, transmit, or broadcast.

Full article, via DVGuru