UUENCODE is derived from the phrase Unix-to-Unix encoding which is a process where a binary file is converted into a text file for easy transmission over the internet over the UUCP (Unix-to-Unix Copy) mail system. UUENCODE was used to convert the data to fairly common characters that were unlikely to be translated and thereby corrupt the file. The matching program called, predictably enough, UUDECODE has the responsibility of reversing the process and recreates the original binary file as it was before encoding took place. This system of encoding and decoding became popular for sending binary files by e-mail and for posting on USENET newsgroups.
UUENCODE has basically been deprecated by newer forms of encoding such as MIME (Multipurpose Internet Mail Extensions) which is useful not only for e-mail, but for all data transmitted over HTTP which requires data to be in a format similar to e-mail too.