Principals are entities that can request SQL Server resources. Like other components of the SQL Server authorization model, principals can be arranged in a hierarchy. The scope of influence of a principal depends on the scope of the definition of the principal: Windows, server, database; and whether the principal is indivisible or a collection. A Windows Login is an example of an indivisible principal, and a Windows Group is an example of a principal that is a collection. Every principal has a security identifier (SID).
Windows Domain Login
Windows Local Login
SQL Server-level principal
SQL Server Login
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.
INFORMATION_SCHEMA and sys
Every database includes two entities that appear as users in catalog views: INFORMATION_SCHEMA and sys. These entities are required by SQL Server. They are not principals, and they cannot be modified or dropped.
Server principals with names enclosed by double hash marks (##) are for internal system use only. The following principals are created from certificates when SQL Server is installed, and should not be deleted.
##MS_SQLResourceSigningCertificate## ##MS_SQLReplicationSigningCertificate## ##MS_SQLAuthenticatorCertificate## ##MS_AgentSigningCertificate##