What is Authentication through Security Modes?

The security mode encompasses the Operation System Security and Application Security levels and provides authentication validation. To authenticate a user accessing the server either locally or remotely, SQL Server uses one of two methods:
1. Windows NT Authentication Mode:- In this mode, SQL Server uses Windows NT authentication to let the user access SQL Server after the user has logged on to the server or the network. SQL Server assumes that the user has already been validated by the operating system, so no further check is performed.
2. Mixed Mode:- In this mode, SQL Server uses either Windows NT authentication or SQL Server authentication. SQL Server first checks to see whether the user is using a SQL Server login account. If so, SQL Server authentication takes place, the password is verified, and the user is granted access to SQL Server. However, if the user is not authenticated with SQL Server authentication, a check is made by using Windows NT authentication to validate access to SQL Server.
It is important to remember that regardless of which security mode is used, any user accessing SQL Server must have an account on SQL Server. This account is called a SQL Server login.