Check your profile, there should be a thing saying: "Hide your online status:" Hidden user is someone registered user who has logged in but who has also decided not to allow others know he's logged in.
post subject This is getting beyond me, if a user is going to bother to log in, then decides not to tell any one that he is logged in, why bother to log in in the first place? If anyone does bother to explain please do not make it too complicated as I am a bear of little brain and all this logging in and out confuses me!
Re: post subject For some it may be a way of preventing people from speaking to them. This will make your visit to the forum less stressful. For others, it's a way of remaining anonymous, because no one can see them being there (except admins - in Italics, Christian ).