MS SQL Server Authentication

(i) Windows Authentication and
(ii) MS SQL Server Authentication. Windows Authentication Mode

When a user connects through a Microsoft Windows ® user account, MS SQL Server validates the account name and password using information in the Windows operating system.

Mixed Mode (Windows Authentication and MS SQL Server Authentication)
Allows users to connect using Windows Authentication or MS SQL Server Authentication. Users who connect through a Microsoft Windows user account can make use of trusted connections (connections validated by Windows) in either Windows Authentication Mode or Mixed Mode. MS SQL Server Authentication is provided for backward compatibility.

Blank Password (not recommended)
If a user attempts to connect to an instance of MS SQL Server providing a blank login name, MS SQL Server uses Windows Authentication. Additionally, if a user attempts to connect to an instance of MS SQL Server configured for Windows Authentication Mode using a specific login, the login is ignored and Windows Authentication is used.