You know how you log onto a computer and you get credentials to do certain things on a computer. Like an admin account can install programs, while a guest account cant.
A domain controller is a means to centralize those user accounts in a network setting. So, instead of setting up a bunch of user accounts on each individual computer, you can set up user accounts on one computer (the domain controller), and just log on to the domain controller from each computer and get your credentials that way.
In a Windows environment, the client could be running Windows XP Pro, and the Domain Controller could be running Windows Server 2003. The XP computer gets joined to the domain during installation or if the administrator is logged onto the computer.
After being joined to a domain, the computer will reboot and instead getting its credentials from within itself, it gets them from the domain controller.
Another quick topic about domains is that after you join the domain and restart your machine (restarting is required to change the computer name or join a domain) you will have 2 choices when you log on to cheeta1. You have a choice to log on to your local machine or the domain. Make sure you know which one your logging into because your username and password will most likely be different when logging into the domain.