The user management menu allows you to create new users. But also lets you edit existing users. The users admin and anonymous are created for a new installation. The admin user, as the name suggests, is the administrator user. The anonymous user is used for people navigating to your website. This means that giving rights to the anonymous user will give permissions for everyone, also those that are not registered in your system. Note that it is not possible to delete users. You can only set them to inactive, which will prevent them from logging in.