Registered Users / Subscribers are people who have a WordPress login. This is straight from the FAQs:
I'm confused, what are all the different types of subscriber?
There are basically only 2 types of subscriber. Public Subscribers and Registered Subscribers.
Public Subscribers have provided their email address for email notification of your new posts. When they enter there address on your site they are sent an email asking them to confirm their request and added to a list of Unconfirmed Subscribers. Once they complete their request by clicking on the link in their email they will become Confirmed Subscribers. They will receive a limited email notification when new post is made or periodically (unless that post is assigned to one of the excluded categories you defined). These Public Subscribers will receive a plaintext email with an excerpt of the post: either the excerpt you created when making the post, the portion of text before a tag (if present), or the first 50 words or so of the post.
Registered Users have registered with your WordPress blog (provided you have enabled this in the core WordPress settings). Registered users of the blog can elect to receive email notifications for specific categories (unless Digest email are select, then it is an opt in or out decision). The Subscribe2->Your Subscriptions menu item will also allow them greater control to select the delivery format (plaintext or HTML), amount of message (excerpt or full post), and the categories to which they want to subscribe. You, the blog owner, have the option (Subscribe2->Settings) to allow registered users to subscribe to your excluded categories or not.