Active Directory Domain Controllers (DC) are servers (Microsoft Windows 2000 server, Microsoft Windows 2003 server or Microsoft Windows 2008 server) that has the Active Directory services installed. When you install Active Directory any of Microsoft Server Operating System, you are promoting it to an Active Directory Domain Controller (DC).
An Active Directory domain should contain atleast one Domain Controller (DC). Additional Domain controllers are required when the network start growing, for load balancing and redundancy.
In an Active Directory domain, each of the domain controllers act as an equal peer, holding a read/write copy of the Active Directory database and they have the capability to send updates to other domain controllers in the Active Directory domain.
An Active Directory domain controller (DC) can be authoritative for only one domain and it is not possible to host multiple domains on a single Domain Controller (DC).
You can promote a Windows 2003 stand-alone server to a Domain Controller by using a wizard named "dcpromo.exe". Click the following link to learn how to install Active Directory.