BACnet is a standardized data communication protocol for Building Automation and Control networks as defined in the ANSI/ASHRAE standard 135 and ISO 16484-5.
The advantage is interoperability between devices of different vendors.
The BACnet protocol defines services to allow communication between devices. Examples include 'Who is', 'I am', 'Who has' and 'I have' for device and object search and identification, “Read Property” and “Write Property” for the exchange of data, up to more complex services for alarm and event management, scheduling and trending.
The BACnet protocol defines a number of object types on which the services operate. Each object is characterized by its properties.
The BACnet objects are combined in a BACnet device. A BACnet device represents the functionality of a physical device.
More background information and introduction can be found here: