From MP3Car.com Wiki
OBD-II is a government mandated diagnostic standard for automobiles for sale in the US starting in the mid 1990s. It is a set of standard protocols and codes that are sent to the in-vehicle modules and responses received. The responses include any diagnostic information as well as sensor data such as RPM, Manifold Air Pressure, Battery Voltage, etc.
The actual communication can take place over J-1850 (SAE Standard bus), CAN (Controller Area Network), ISO 9141, or other standard automotive bus. The SAE specifies a standard (J1962) connector with 16 pins for this purpose - pin 16 is battery power, and pins 4 and 5 are chassis and signal grounds. Pin 2 is normally J1850, and other pins have default assignments.
OBD-II also allows for manufacturer specific extensions and does NOT include normal inter-module bus traffic. Normally speed, RPM, odometry and often even commands like lock or unlock doors travels over the bus, but in a proprietary (but often easily decodable) format, not OBD-II.
