"alt." is the prefix for any of the hundreds of "alternate" user-originated newsgroup s that are part of Usenet . Like other newsgroups, each "alt." newsgroup is arranged in a hierarchy of topical discussion boards that you may read or post to. "alt" is one of many major newsgroups; others include: news, rec (recreation), comp (computers), and soc (social). The "alt" newsgroups are known (and used most frequently) for their alt.sex and related categories, but are also known for including a wide and inventive range of discussion topics.
The easiest way to get access to newsgroups is through the Google Web site.