A database username is a name known to the database, and assigned to a login ID for the purpose of allowing a user to have access to a database. A database username is often simply called a user. The abilities a user has within a database depend on the permissions granted to the username, and the permissions granted to any groups the username is a member of.
The user must already have a login ID on a SQL Server. You must add a login ID for the user on a SQL Server before you can add the username to a database. If you add a new user to a database without specifying a username, the username is automatically the same as the login ID.
Because usernames are separate from login IDs, user preferences can be accommodated. For example, if there are five SQL Server users named Mary, all of them will have different login IDs. However, if they don't use the same databases, each Mary might prefer to be known simply as mary inside a particular database.
Users who have been given access to a database still need permission to do things inside it. These permissions are set up by the database owner or the database object owner. For information about permissions, see About Permissions.