Although used interchangeably in most references, but in SQL Server, a Login and a User are two different things. You first create a Login identified by a password at SQL Server level. You can then create a user at database level that will be mapped to login.
Access permissions to database objects and roles within the database are granted to the user. While server roles are granted to Logins.

Please Note: To connect to a database, a login must be mapped to a database user.

If your login is not mapped to any database user, you can still connect to SQL Server instance using SQL Server Management Studio (SSMS), but you’re not allowed to access any objects in the database. 

Creating a Login and User using SSMS

Let’s create a login first with a name “DBA_USER”. Follow the screenshots as shown below:

Open SQL Server Management Studio and connect to the SQL Server instance.
Under Object Explorer > SQL Server Instance > Logins, right click as below:

Enter Login name and password for the login as below:

Optionally, grant appropriate server roles.

Map the login with username created in database, if the user has been created already.

Your login has been created.

Now, let’s create a user with the same name “ravi” as shown in below screenshots.
Please Note: Users are created by expanding the database and then expanding security and right clicking on User.

The newly created user can be seen below:

Creating a Login and a User using T-SQL

/ ***** Object: Login [ravi] *****/
USE [master]
GO

CREATE LOGIN [ravi] WITH PASSWORD=’somethingsecret’, DEFAULT_DATABASE=[master]
GO

ALTER LOGIN [ravi] ENABLE
GO

ALTER SERVER ROLE [sysadmin] ADD MEMBER [ravi]
GO

/**** Object: User [ravi] *****/
USE [ravidb1]
GO

CREATE USER [ravi] FOR LOGIN [ravi] WITH DEFAULT_SCHEMA=[dbo]
GO