A database user is a principal at the database level. Every database user is a member of the public role.
When a database is created, the database includes a guest user by default. Permissions granted to the guest user are inherited by users that do not have a user account in the database.
The guest user cannot be dropped, but it can be disabled by revoking its CONNECT permission. The CONNECT permission can be revoked by executing REVOKE CONNECT FROM GUEST within any database other than master or tempdb.