CAVLC, which stands for Context-adaptive variable-length coding, is a video encoding method used in MPEG-4 AVC. Compared to the alternative, CABAC, it takes less time to encode and can be played on slower hardware. However, it's less efficient at lower bitrates.
Although less complex than CABAC, CAVLC is still superior to techniques used in MPEG-4 ASP codecs like DivX and XviD. These older formats used fixed length encoding instead, which distributes bits more evenly across all the pixels in a frame. By comparison, variable length coding can allow some pixels to have more information than others. This makes the quality throughout each frame more even as more detailed areas can be described with more bits.
CAVLC is supported by all MPEG-4 software and hardware decoders, including those found in HD DVD and Blu-ray players.