Every user has a primary email address, which is the email address created when the user’s account was created and the address the user uses to log into Google Apps. Google Apps enables to you create additional email aliases for a user.
An email alias is an alternative email address that points to an existing user account. For example, the user email@example.com might also want to receive email addressed to firstname.lastname@example.org. If you create email@example.com as an email alias for firstname.lastname@example.org, Google Apps will deliver email for both of these addresses to the same Gmail inbox.
To set one up simply login to your mail account at http://mail.yourdomain.com substituting yourdomain.com for your domain.
This will instantly setup your nickname and you’ll now be able to receive e-mail at both these email addresses in the same inbox. You only get 1 nickname per e-mail address. You can make groups to have a e-mail address forward to multiple e-mail addresses.