ACD - Automatic Call Distribution - a single number(s) that customers call that goes to a group of agents (ACD Agents) so they can help the caller. This can be used for sales, after sales service, Help Desk scenarios etc.
This can be simple or complex, depending on your set up. Simple = Call rings until a logged in agent is available to take the call and the system sends it to that person. Complex = through Call Center software/ voice mail "tree" to offer the caller options for automated or manual help, typically resulting in being answered by and ACD agent if automatic systems cannot help.
Is this what you are looking for, or a technical set up description?