Fixed database roles are defined at the database level and exist in each database. Members of the db_owner and db_securityadmin database roles can manage fixed database role membership; however, only members of the db_owner database role can add members to the db_owner fixed database role.
The fixed database roles are the following:
public Database Role
Every database user belongs to the public database role. When a user has not been granted or denied specific permissions on a securable, the user inherits the permissions granted to public on that securable.