In the Real World, Usenet has been around since 1979, long before the government computer system known as ARPAnet became the modern Internet. It was the original online community, and the origin of many technological developments and much of the jargon of the Net today!
See also newsgroup.
Within the continuity of the LNH, Usenet is a cosmology of world-newsgroups, also known as the Usenetverse. (The Omnilooniverse is a subset of the Usenetverse, centered within RACC and a.c.lnh.) The RACelestials are mysterious cosmic beings that have something to do with its structure and functions, and the Xinerama Brotherhood claim to be protectors of the Usenetverse.
The Existents claim that the Usenetverse is actually the collection of all stories that have been or will be posted in the LNH imprints, and that the worlds traveled to are but Omnilooniversal derivates of the newsgroups in question; however, it's been hinted that the truth may be even more complex.