by 
The Curious Bag
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Automotive electronics have gone through several incarnations over the years. Modern GM computer controls serve two main functions: providing output signals to run the vehicle's vital systems, and collecting information from the sensor array to diagnose drive-train malfunctions.
History
OBD-I (On-Board Diagnostics, version 1) was the standard programming format for all computer controlled cars sold in the United States until 1996, when it was replaced by OBD-II.
OBD Software Differences
OBD-II software was less an evolution of OBD-I than it was an industry-wide standardization of code language. Before 1996, each make used a different set of codes to diagnose engine problems, making it very difficult for independent shops to service new cars.
OBD Hardware Differences
GM OBD-II hardware is more advanced in every way than OBD-I. Faster computer processors and a more comprehensive sensor array (including a second oxygen sensor) make OBD-II systems far more advanced in terms of diagnostics.
System Functions
Computers with OBD programming run many systems, including engine air/fuel/spark control, emissions equipment, transmission shifting and antilock brakes.
Code Scanning
Thanks to code standardization, any "check engine" light on any new GM car can be diagnosed with a generic scanner that plugs into a socket beneath the dashboard.
Source:
Resource:
OBD-II Scanners Explained (Video)
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