MIME stands for Multipurpose Internet Mail Extensions. The need for MIME arose because only simple text documents could be sent via e-mail. To send other documents (images, compressed archives, etc.), it is necessary to first convert them into text-only data. Once the document is mailed, the receiving software needs some way to know what it was sent.
MIME types are used to identify what the content sent really is, regardless of what form it was mailed in.
On the web it is not necessary to convert data to a text format, but MIME types are still used to tell the browser what form the data is in.