Difference between sql logins and sql users in sql server :-

1 A "Login" grants the principal entry into the SERVER and A "User" grants a login entry into a single DATABASE.

2 SQL Server Login is for Authentication and SQL Server User is for Authorization.

Authentication can decide if we have permissions to access the server or not and Authorization decides what are different operations we can do in a database. Login are created at the SQL Server instance level and User is created at SQL Server database level. We can have multiple user from different database connected to a single login to a server.

3 Logins will have the access of the server and Users will have the access of the database.

Link

https://www.isunshare.com/sql-server/difference-between-sql-server-login-and-database-user.html

https://www.top-password.com/blog/difference-between-sql-server-logins-and-database-users/