OBD is an abbreviation of On Board Diagnostics. The main current standard is OBD-II or OBD2. This is a standardized form of self-diagnostics and reporting that can be found in all cars and light trucks that were built during or after the 1996 model year. The primary function of an OBD-II system is to gather diagnosic information that can be used to monitor and repair vehicles.
OBD-II is an improvement over OBD-I in both capability and standardization. The OBD-II standard specifies the type of diagnostic connector and its pinout, the electrical signalling protocols available, and the messaging format. It also provides a candidate list of vehicle parameters to monitor along with how to encode the data for each. There is a pin in the connector that provides power for the scan tool from the vehicle battery, which eliminates the need to connect a scan tool to a power source separately.