Authentication means identifying a user based on its username and password.
Two authentication modes on SQL Server are :
Windows Authentication : It is the default authentication mode in SQL Server. Trusted user and group accounts are authenticated when they login to the system. They do not have to present any additional credentials.
Mixed Authentication : It supports Windows authentication as well as SQL Server authentication. Windows authentication is the same as above. SQL Server maintains a username and password for authentication of valid users.
You can choose an authentication mode by changing Server Authentication on Security page in Properties of SQL Server Management Studio.
