4:4:4 and 4:2:2 are shorthand experessions for digital video standards. The numbers specifially refer to the sampling frequency ratios for, respectively, the luminance and color difference components (Y, R-Y, B-Y) of a component video signal
Thus a 4:4:4 format takes twice as many chrominance samples per unit time as a 4:2:2. Generally, 4:2:2 is considered better than the average person can see (and so really the "standard" for high-quality sampling. 4:4:4 is the "gold standard" of sampling and you'll usually only see it in really high end studio-type systems or systems where the engineer knows that she / he is going to do a LOT of fiddiling and converting later.
This is all described in an absolutely riveting technical specification known, endearingly, as ITU-R BT.601-5, and available at: http://www.itu.int/home/index.html (for a small fee.)
HINT: The most important reason for you to pay attention to this spec is that 4:2:2 is (at least IMHO) the lowest you'll want to go if you intend to do any downstream chroma-keying with your video. 4:1:1 won't chroma-key well.