To flesh out the answer to this question, a "Registered User" is a member who has not confirmed the email address associated with their account. This most frequently appears with new members who haven't posted and never responded to the activation emails sent to them when they first signed up.
But it also happens when a user changes the email address associated with their account. When they do that, they have to go through the activation process, and until they click that link to activate with the new email address, they show up as a "Registered User" instead of with a normal title.