Lurker and Lurking
A lurker is a user of an forum who reads content but does not post to the forum.
It is usually considered wise for a newcomer to lurk for a period of time before posting to the forum. How much time one should spend lurking depends upon the forum.
There are also some who lurk on a forum habitually, and rarely if ever contribute. It is generally difficult to guess how many such lurkers are present, since by definition they are silent.
lurking – The activity of one of the “silent majority” in a electronic forum such as Usenet; posting occasionally or not at all but reading the group’s postings regularly. This term is not pejorative and indeed is casually used reflexively: “Oh, I’m just lurking”. Often used in “the lurkers”, the hypothetical audience for the group’s flamage-emitting regulars.
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Source: http://www.thefreedictionary.com