The act of being with a group of people but not saying much. Usually these people (who don't say much) are totally happy with that fact and they just like to listen and don't feel the need to be the center of attention.
The act of being signed into gmail chat but putting your status to invisible. Lurking involves watching friends sign in and enjoying knowing that they dont know you're there. Having too much time on one's hands is the most common cause of lurking.
The act of going to a night club or busy bar and scoping out your prey in the shadows. Then, when they least expect it -or when they are at their highest point of intoxication-, you swoop in and get really fashion friendly. Usually the natural habitat of a lurker is around the dancefloor.
Pitman: What is Bean doing over therein the shadows?
Hunter: Looks like he is lurking again.
Ham: He is scoping out prey to be his next victim...lurking is an artform.