Depending on how your website permissions are setup, users may be able to register a new account, login using an existing account, or view or edit their account settings.
For security reasons, anonymous visitors have limited access. They are typically allowed to view some content and may even be allowed to add comments, however all administrative functions are typically hidden from them.
Users with administrative permissions may be able to create and delete content, adjust site features, view logs, delete other user accounts, etc. Exactly what each set of users can do is adjusted by assigning permissions for each Role.