The standard for attaching non-text files to standard Internet mail messages. Non-text files include graphics, spreadsheets, formatted word-processor documents, sound files, etc.
An email program is said to be MIME Compliant if it can both send and receive files using the MIME standard.
When non-text files are sent using the MIME standard they are converted (encoded) into text - although the resulting text is not really readable.
Generally speaking the MIME standard is a way of specifying both the type of file being sent (e.g. a Quicktime video file), and the method that should be used to turn it back into its original form.
Besides email software, the MIME standard is also universally used by Web Servers to identify the files they are sending to Web Clients, in this way new file formats can be accommodated simply by updating the Browsers' list of pairs of MIME-Types and appropriate software for handling each type.