Motion JPEG (MJPEG) is a video codec where each frame is separately compressed into a JPEG image.
The resulting streams quality is independent from the motion in the image, different from MPEG video, where quality often decreases when small images move rapidely.
Data rate is between so called uncompressed formats like RGB (compression 1:1) and YUV (compression 1:1.5...1:2.5) and MPEG (which uses similarities between images).
For typical data rates (29 Mbit/s) quality is very high, files are very large.
Beside YUV it's mainly used by high-end frame grabber PC boards, like the AV Master. It is also used by digital video cameras DVC.