UUCP is an abbreviation for Unix-to-Unix Copy. It started out as a package of programs to transfer files over serial lines, schedule those transfers, and initiate execution of programs on remote sites. It has undergone major changes since its first implementation in the late seventies, but is still rather spartan in the services it offers. Its main application is still in wide-area networks based on dial-up telephone links.
UUCP was first developed by Bell Laboratories in 1977 for communication between their Unix-development sites. In mid-1978, this network already connected over 80 sites. It was running email as an application, as well as remote printing. However, the system's central use was in distributing new software and bugfixes. Today, UUCP is not confined to the environment anymore. There are both free and commercial ports available for a variety of platforms, including AmigaOS, DOS, Atari's TOS, etc.
One of the main disadvantages of UUCP networks is their low bandwidth. On one hand, telephone equipment places a tight limit on the maximum transfer rate. On the other hand, UUCP links are rarely permanent connections; instead, hosts rather dial up each other at regular intervals. Hence, most of the time it takes a mail message to travel a UUCP network it sits idly on some host's disk, awaiting the next time a connection is established.
Despite these limitations, there are still many UUCP networks operating all over the world, run mainly by hobbyists, which offer private users network access at reasonable prices. The main reason for the popularity of UUCP is that it is dirt cheap compared to having your computer connected to The Big Internet Cable. To make your computer a UUCP node, all you need is a modem, a working UUCP implementation, and another UUCP node that is willing to feed you mail and news.