The .MJPEG, or .MJPG, format contains a series of .JPEG files in sequence. The file works as a video file as a result. Each individual .JPG in the file is compressed and doesn't lose quality when the motion in the whole video changes quickly. Webcams and other mobile cameras tend to use the .MJPEG/.MJPG file format. The way the files are compressed results in more efficient processor use but can use up noticeable disk space. You can use Mozilla Firefox or Apple's QuickTime Player to open the files. The .MJPEG, or .MJPG, format name stands for motion .JPEG, a series of .JPEG files in sequence.