MIME emails enable you to send attachments (picture files, binary files, and so on) as opposed to text-only emails.
for example, if you used any email program (not text-based) and attached a picture to it, the email would be MIME-encoded.
to try this for yourself, send an email with an attachment to yourself, then use the email program to view the source of the message. if you're using netscape mail, do a view source, if you're using outlook, select File->Properties, and then there will be a button to see the entire message (can't remember what's written on the button).
have a nice day!
