Users with asterisks by their name have enabled the 'Hide me from the Who's Online list' option. New and non-upgraded members cannot see their username on the list. Upgraded users, however, have the ability to see all users that are online even if they choose to be hidden, therefore the asterisk is implemented so users can easily differentiate between hidden and non-hidden users.
I hate to be a pain the ass, but if you bothered using the search feature for its primary purpose, you would have quickly found your answer without having to waste any effort making a thread.