Before, you can access anything on SQL Server, apart from knowing what to call the server, the server itself needs to know who you are. This process is called authentication.
In SQL Server, a login is person that the server has identified.
Now access to the server by a login doesn't mean that you can access anything much on the server, it's just the first step where you get the server to trust who you are.
SQL server has 2 security modes.
In one mode (called Windows mode), SQL Server trusts the Windows operating system about who you are. You've already proven to Windows who you are, SQL Server trusts Windows, and so it trusts who you are, from your Windows login.
In the other mode (called mixed mode), SQL Server not only trust Windows logins, but it is willing to trust a list of logins that it manages itself, rather than just the logins that Windows manages. It records its own set of usernames, and passwords and uses those for authentication. (It doesn't store the password, it just stores a hash of the password, for security). The usernames, and password hashes are stored in the system master database.
Once you have connected to the server, you are considered to have "logged on", and you are considered to be a "login" by SQL Server.
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