"MIME (Multipurpose Internet Mail Extensions) is a standard to send stuff other than 7Bit ASCII by e-mail." (Claus Aßmann)

The standard is composed of three parts:

• Part One: Format of Internet Message Bodies -- RFC 2045 [1]
• Part Two: Media Types -- RFC 2046 [2]
• Part Three: Message Header Extensions for Non-ASCII Text -- RFC 2047 [3]

It started as an extension to [RFC 822] [4], which describes internet mail, but has a much broader usage.

DKF: It's particularly important for smtp and http, but finds good use with nntp (though not usually the attachment parts there).

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