Usenet (Users' Network) is a bulletin board shared among many computer systems around the world. Usenet is a logical network, sitting on top of several physical networks, including UUCP, BITNET, and the Internet. Sites on Usenet include many universities, private companies, and research organizations. Current estimates indicate that there are over two million participants at over 45,000 Usenet sites in over fifty countries, with more users and sites joining every day. Most are running the UNIX operating system, although the number of non-UNIX sites, e.g. VMS sites and PCs, is increasing.