Base64 is a method of encoding
binary data into a form of pure
text that can be (for example) safely
e-mailed. It is intended to be the mechanism for
encoding file attatchments to e-mail using
MIME, and is generally thought of (coupled with MIME) as a newer alternative to
uuencode. Its
internal workings are described in detail in part 5.2 of
RFC 1521.
Most modern mail clients will transparently encode and decode base64, so you don't really need to be aware of the fact it exists.