What is the format for logging into a domain account in OS X? At the login screen, how do I login to a domain account?

For example, on Windows the username might be something like louis@contoso.local, where the domain is contoso.local.

Edit: The AD username happens to be the same as a local username.
You need to set up Network Account Server settings in Preferences > Users &amp; Groups > Login Options.

In the company I work for I can either use DOMAIN\username or username@DOMAIN when logging in to the AD on from a Windows Machine, but on the Mac this is not required. I just type in username and my AD password, and it logs me in.