Who is online is probably a little mis-named. It actually refers to who is currently visiting the forum. It is accurate in that it is updated on every forum page load – including which forum/topic a user has loaded.

Users are removed from the list when they either (a) logout or (b) do not load another forum page after a set number of minutes –  which you set in the forum admin options.

There has been some talk about possibly starting to include visitors to the site who do not visit the forum but nothing concrete on that score as yet.