A lurker is someone that follows the forum but doesn’t post.

someone who seems to always be present bu unnoticed

one who reads a blog without ever commenting on the posts

A person who is just "there." Nobody really knows why, other than they show up and don't say a word. Possible reasons for this could be:
-They don't know what to say
-They are pretending to have friends
-They are trying to scare you
-They are there just to be seen with you, as if they have some kind of relationship with you.

a person who reads the posts on a forum, message board, etc, but does not post

Lurkers lurk. This lurking may exist in the form of excessive monitoring and creating of PT threads, checking out other people's myspaces without the intention of leaving a comment or message, or hitting on/trying to get with someone. Most people are a lurker sometimes, but you aren't truly a lurker until PT or myspace is used everytime you see a computer, or you're creepy (possibly old) and hit on mad girls.

A guy who hangs around your girlfriend as a "friend" who is really just waiting for the day when you and your significant other are on the rocks. That is when he swoops in to make a move, as the comforter, supporter, and basher of her current bf. Women are oblivious to them entirely.