A "virtual" or simulated user is actually a thread started by AppPerfect Load Test to throw requests at your web service/application or database. The virtual user acts like a real user and performs the same actions as a real user would perform. You can define a large number of virtual users in your test.
Virtual users can be either named or anonymous. If your application requires all users to log in (via a username/password), then you will most likely need to use named users. In other cases, you can use anonymous/generic virtual users.
While internally AppPerfect Load Test handles a named user the same way it would handle any other parameter, for better usability, virtual user management is separated out from the rest of the parameter management UI.