A Lurker is the internet equivalent of that creepy guy at the party who stands in the corner all night, listening to other people's conversations, and no one can remember who invited him. Except somewhat less creepy (as you don't see him).
Some lurkers will sign up for a forum but rarely post anything even though they read most of what does get posted. Other lurkers, on publicly viewable forums, won't even sign up.
Sometimes lurking is a prelude to becoming a regular poster on the forum. Either the lurker likes what they see and decides to become an active part of the community, or they were intending to join all along and merely lurked to get a handle on the unspoken rules and zeitgeist of the forum. Thus, a period of lurking can help one bypass the noob stage of the forum poster life cycle. Lurking for this purpose is strongly encouraged as a survival mechanism on certain meme-heavy corners of the Internet, hence the phrase "Lurk moar". Then there are pay-for-entry forums, which means that there will be people interested on what happens on the forum but unable or unwilling to sign up due to the cost.
However, if one admits to formerly being a lurker, the registered users might be creeped out that the information they posted on a publicly viewable forum is being viewed by the public.
Related to the Unknown Troper.
Some sites, such as 4chan, will actually suggest that you should "lurk moar" if you prove ignorant of the etiquette, such as it is.
Sometimes the reason for only lurking on a TV-show's forum is that by the time the show airs in the lurker's country, everything useful has already been said.
While lurking is encouraged in these latter days of always-on broadband, such was not always the case. The term was first used in its present sense in the 1980s when the web didn't exist, and the Internet was still confined to governments and universities. People would connect their Commodore 64s and IBM compatible computers to bulletin board systems via modem. These were often hosted by fellow geeks in their own homes, and usually used a modem connected to a single phone line, meaning only one user could be on at a time. Finally, BBS's were often only online for certain hours each day. Thus, a lurker was someone who tied up the phone line without contributing to the community.
- In The Destroyer, Dr. Smith is the original lurker. The assumption presented in the later books is that Mark Howard follows in these footsteps.
- The Legion of Net.Heroes has a lot of heroes and villains with lurking based powers such as Lurking Girl, her evil clone Lurking Lass, Lurker Lad, and Netlurker.