Until recently, email could transmit only plain English text. Words in other languages, with characters that don't belong to the character set used in English, couldn't be sent. For instance, Spanish-speaking email users had to use kludges like monta~na to stand for the word montaña. If you wanted to send data, pictures, word-processor documents, sound, video, and other non-text messages, you had to use workarounds that many recipients couldn't handle.
In 1992, the Multipurpose Internet Mail Extensions, MIME, started to change that. The MIME standard lets users around the world send text and documents in a way that everyone can receive. If a recipient's email user agent can't display MIME messages, the messages will still be in a format that makes most of the text readable. Instead of trying to change everyones' mail transport systems, MIME was designed to work around limitations -- to work with existing systems.
More and more email user agents support MIME. MH support started with a set of patches to MH version 6.7.2. Both exmh and MH-E can send and receive MIME mail. If you use xmh, there are patches in the Metamail package for MIME support.
Another Chapter has more information about MIME.