Im retarted but help about OBD0 + OBD1.

I have no clue what OBD1 is. Im sorry but im really stupid, I see it everywhere and im just learning about cars and stuff, but I cant figure out what it is. So tell me what are these things? If i had to guess, im guessing the different kinds of ECU's? Thats just from the top of my head, help me out there.

obd= on board diagnostics 0,1,2 are just what level of obd you are running. yes the ecu's are different. the higher the # the better the emission controls are. however this is usually a bad thing as it can hinder performance. i prefer obd0 myself although IMO obd1 would be the best as far as tuning is concerned.