XenuPlease be aware that Wikipedia is an attempt to write an encyclopedia collaboratively online, and that therefore Wikipedia contains spoilers in its discussions of films, novels, etc., as well as in its discussion of 'secret' religious writings.
Xenu is a figure associated with the doctrines of Scientology. He (or it) is included in the "secret" teachings of Scientology, which are only revealed to upper-level Scientologists. The documents involving Xenu are included in the upper-level Scientology course known as OT III ("Operating Thetan Level Three"). For more information about the "secret" documents of Scientology, see the Wikipedia article on Scientology beliefs and practices.
In spite of efforts by Scientology to keep the story of Xenu a secret, details of the Xenu writings have been revealed to the general public over the years. Critics of Scientology point to the Xenu story as ultimate proof that Scientology is a scam and a confidence trick. They note that after spending thousands of dollars and investing years of their lives to reach the level of OT III, Scientologists learn that the true origins of human existence are based on a story that appears to be, to novices, badly written science fiction.
An ex-Scientologist, Roland Rashleigh-Berry, wrote a "Xenu leaflet" that, according to him, reveals many of the details of Scientology's secret teachings.
The Church of Scientology maintains that viewing the OT III materials (the scripture in which Xenu is mentioned) without proper preparation and supervision is likely to cause death by pneumonia. In its public statements, Scientology has been notably reluctant to admit the existence of the Xenu writings. In the instances where it has acknowledged them, the Church has stated that the origin story of Xenu is a religious writing and can be seen as the equivalent of the Old Testament, in which certain miraculous events are described that seen unlikely to have occurred in real life.
See also: Scientology vs. the Internet
Xenu is also software that checks web sites for broken links. Link verification is done on "normal" links, images, frames, plug-ins, backgrounds, local image maps, style sheets, scripts and java applets. It displays a continuously updated list of URLs which can be sorted according to different criteria.