Email was designed to be a text-only medium, and various tricks need to be used in order to send any other type of data. You've probably already come across attachments, which allow you to package a file or files within the email. The file is encoded so that it won't become corrupted when passing through the email system, in one of several formats:
UUEncoded — outdated and not very common now;
BinHexed — common on Apple Macintosh computers
MIME — Multipurpose Internet Mail Extension — the almost universal standard for the Internet.
If you have a choice of formats to encode your attachment, MIME is almost always the best choice. If someone sends you an attachment in a format that your mail client can't support, then there are free tools available from most shareware/freeware websites that can decode them for you.