First of all: YUV pixel formats and Recommended 8-Bit YUV Formats for Video Rendering. Chromium's source code contains good documentation about those formats too: chromium/src/media/base/video_types.h and chromium/src/media/base/video_frame.cc (search for RequiresEvenSizeAllocation(), NumPlanes() and those kinds of functions).
You can think of an image as a superposition of several planes (or layers in a more natural language). YUV formats have three planes: Y, U, and V.
Y is the luma plane, and can be seen as the image as grayscale. U and V are reffered to as the chroma planes, which are basically the colours. All the YUV formats have these three planes, and differ by the different orderings of them.