PBX (Private Branch eXchange)
A phone switch located at the customer's premise. A PBX comes in many sizes, from 2 lines to thousands of phone lines. Common features can include an ACD for call distribution and IVR that can process incoming phone calls prior to routing to call stations. Some PBX's support outbound dialing protocals such as autodialing, progressive dialing and predictive dialing. A PBX usually has many more features than a standard phone system.
A Hosted PBX (sometimes called a Virtual PBX) offers similar functionality and features, but the switching is located a central location and only the phones are at the customer site.