What does the 420 means in YUV420?
The 3-digit number describes the subsampling of the
chroma (U and V) channels.  A detailed explanation is at

http://en.wikipedia.org/wiki/Chroma_subsampling

In particular, YUV420 means that the U and V channels have half the resolution of the luma (Y) channel, in the horizontal and vertical directions. The sampling method usually present in low to medium quality JPEG images is YCrCb420 (high quality JPEGs are usually YCrCb444 (no subsampling).