On Board Diagnostics (OBD) is a term for vehicles referring to a cars reporting and self diagnostic (on board diagnostic) capability. The OBD is what allows us to access and check the state of your vehicles information and various sub-systems. Since OBD was first introduced in the early 1980's with on board vehicle computers the diagnostic information from On Board Diagnostic systems has varied widely as it has grown in time making OBD possible.

Early OBD Stages:
The early stages of OBD was very simple meaning that all they would do was light a Malfunction Indicator Light (MIL) when detecting a problem. They would not provide what the problem was or any information about its error.

Modern OBD Stages:
To identify and fix malfunctions and problems within a vehicle todays modern On Board Diagnostics use a fast standard digital com port which can give us real time data in addition to a series of standard diagnostic trouble codes (DTCs).

Also See below for more information:
ECU Remapping
OBD Remapping
Chip Tuning
Diesel Tuning Kits
Wikipedia On-Board Diagnostics
Wikipedia Table of OBD-II Codes