More advanced video encoding standards like AVC (MPEG-4 Part 10), allow the division of Macroblocks into smaller blocks called Partitions. This allows large areas to still be encoded efficiently in full Macroblocks, while Macroblocks with more varied detail can be encoded Partitions to improve quality.
Since most consumer video is encoded using the YV12 Colorspace, a 4x4 Partition contains 16 Luma samples and 4 U and V (chroma) samples.
Partitions are most important for high definition video, and especially when it's viewed on large displays. Video encoded with only full Macroblock DCT, rather than Macroblock and Partition DCT are more prone to show Blocking artifacts.