With all your telephony needs conveniently available through in a web browser, it raises the question what sets an ACD System apart form just a standard IP based telephone system?
In general terms, an ACD system is geared toward call centers - serving mostly call agents, while a standard phone system is geared toward a general use of a company serving all users. In a standard phone system we refer to departments as Groups. In an ACD system we refer to departments as Queues.
Because of this, the nature of call flow in a standard phone system is very straight forward. Clients call users directly on their extensions or call the main number of a the company, hear a greeting, and get transfered to a department they require. Any user from this department can answer the call or it will be go to a voice mail if no one picks up.
In an ACD sysetm, clients can call agents directly on their extensions. But if they call the main number, the ACD kicks in and distribute their calls according certain rules, priorities, and predefined settings. For example, if a client chooses to be transfered to the Sales department, the ACD may assign the call to an agent with the highest priority or it may assign the call to an agent that has not received calls in a while. If all agents in the Sales department are busy, the call will be transfered to another department based on priority ranking of the different departments.
You can refer to this article for more information.