All luminance information is retained. Thus, there are 2 bits per pixel for any of U and V information. This subsampling does not drastically affect quality because the eye is more sensitive to luminance than to chrominance information. Subsampling is a lossy step. In MPEG, video stream breaks sequence of images into a series of layers, each containing more precise sample clusters.
Picture is primary coding unit of a video sequence.
Being rectangular, the Y matrix has an even number of rows and columns. Slice -MPEG picture is composed of slices where each slice is sequence of macroblocks in raster scan order. Slices can strech from one macroblock row to the next. This slice structure, allows great flexibility in error handling and discovering changes in coding parameters. Macroblock is basic coding block of an MPEG picture.