A login exists on the server-level and gives you access to the SQL Server but not necessary to a database.
A user exists within a database and has access to that database.
Under normal conditions a user must be linked to a login.
In SQL 2005/2008 it's possible to create a user without a login. But this is a special case mostly used for service broker communication.
[font="Verdana"]Markus Bohse[/font]