On board diagnostics

Please explain me what is On board diagnostics OBD II. I know it is diagnostics. But my doubt is what all components in the vehicle i need to check to report diagnostics?

oBD2 is a protocol, as a rule all the control modules in a vehicle are connected to it, you can get free software to do generic obd stuff, if you want to look at abs, airbags, tipm's etc then you need some specialized software or the manufacts software.
What vehicle are you dealing with?