On Board Diagnostics
OBD (On Board Diagnostics); Please see Wikipedia's On Board Diagnostics article for detailed information. This article is intended to be both an abridged and expanded version of that article with a focus on practice rather than theory. IE, if it's your intent to apply this to a car you're working on then this article is for you.
A Brief History of OBD, OBD-II, CAN, and Others
The earliest version of OBD was introduced by Volkswagen on their Type III vehicles. In the 1970s and 1980s most every manufacturer had some basic implementation of OBD, most of which were proprietary. It wasn't until the early 1990s that an OBD standard emerged. Then in 1996 California required OBD-II to be included with all vehicles sold in the state. In 2008 all US vehicles are required to implement the CAN (Controller Area Network) standard (well, technically a similar version titled ISO 15765-4). OBD-II standards continue to implemented on 2008 and later vehicles that implement the CAN standard. OBD and proprietary standards are considered 'legacy'.
OBD-II is a 'one way' protocol in that information is only transmitted from the car to the scanning instrument. Many manufactures use the OBD-II interface to allow programming of various vehicle components with proprietary protocols.
The CAN bus is functionally equivalent to Ethernet network for PCs and Macs, although the protocols and physical infrastructure are radically different. Basically it allows communication between sensors and 'intelligent' components on the bus (equivalent to a computer network).
Physical Components (as it relates to how most people will use it)
'Connector' (on the vehicle); OBD-II connectors are pink in color and located underneath the dashboard on the driver side foot well of the vehicle, closest to the door. This is a requirement of the OBD-II standard. It is a female connector.
'Interface'; This is either a cable a cable integrated with a scanning device. Cables designed for use with computers have a male version of the OBD-II connector on one end and usually a USB connector on the other with older versions having a 9 pin serial connector. There are also more modern versions that are wireless and use blue tooth.
'Scanner'; This can be a self contained physical unit or software on a PC, Mac, or Smart Phone.