In this user profile after their name is the tag "Unregistered".
What does it mean, and what is different in comparison with a common (registered) user?
You do not need to register in order to participate here (on other Stack Exchange sites it may be different); just leaving a display name and email address is enough. You can test this yourself by opening a private browser window (or another browser) and start writing a question:
The system will remember you through cookies, but once you clear those, you're effectively logged out forever, meaning you can't edit your posts, reply to comments or upvote. Unregistered users are also not shown in the list of users.
For more information, see the questions about 'unregistered users' on Meta Stack Exchange.