Auto-Attendant is a particular simplified application of IVR technology with the function of a virtual receptionist. Its main purpose is to greet callers and transfer them to extensions or departments in a company without involving assistance from a human receptionist.
An Auto-Attendant will typically prompt a caller to "enter an extension" to be connected or to make a choice from a menu system - "for sales, press 1, for customer service, press 2," etc. It may also offer the option to be assisted by a live operator by dialing a number, usually "0". A dial-by-name directory is usually also available as to allow callers to locate particular employees or users.
Auto-Attendant and IVR are terms that are often confused to be the same thing. However there is a distinction between the two in the sense that an IVR can perform more advanced functions and provide a more tailored interaction that an Auto-Attendant cannot. Where Auto-Attendant simply follows menu choices, IVR provide interaction based on a callers voice response or details he enters on his key pad.
In addition to routing callers to Auto-Attendant can transfer callers to Voicemail or play informative messages (i.e., "Our opening hours are ..."). More advance Auto-Attendant come with the ability to route calls to destination outside the company such as mobile phones, VoIP virtual phones, and other IVR systems.