MJPEG is short for Motion-JPEG which is a means of storing video footage from IP cameras and digital camcorders.
An MJPEG Movie consists of many JPEG images, one after another. Since JPEG is a compressed format, so too is MJPEG, providing a low file size when compared to image dimensions. Each frame of an MJPEG movie has no reference to the next, so inter-frame compression is not used, making MJPEG movies larger than similar MPEG movies. However, since there is no inter-frame compression the file size of an MJPEG movie will not change dependent on the complexity of the images used. This means the file size is a lot easier to estimate when looking at recording solutions.
MJPEG, being composed entirely of JPEG images, is a video-only format. Should you require audio recording you may wish to use another codec such as MPEG or H.264.