The main component of the authentication in SQL Server is as below.
Securables : The securable resources in the Server, Database and Schema categories for which SQL Server grants access
Server category: Server roles, logins, databases, endpoints, availability groups and server roles etc
Database : Database user, role, schema, application roles, schema, certificate, full-text catalog etc
Schema : Table, procedure, function, view, synonym
Permissions : The permissions control actions we assign or deny for accessing the securable
Principals : The principals are the entities, such as logins, database users to which we assign the permissions