The MIME protocol is a standard used to include (or attach) additional information to a mail message. That additional information may be an ASCII file, a binary file, a video clip, an audio file, an image, or another object.
A MIME compliant mail program will follow the directives embeded in the mail message to prompt the user to save the attachement as a file, play the video/audio clip, or perform a certain function according to the information stored in the attachement.
Not all mail reader programs are MIME-compliant.