Hi Katina: In '82 AMG was an independent tuner and manufacturer/retailer of performance parts. You could send your MB directly to them in Germany for mods or buy the parts and DYI. The mods could have been full engine, suspension and aerodynamics or just a few cosmetic body parts.
AMG did limited series production (if a few dozen vehicles can be considered "series") and offered full meal deal upgrades to individual customers, in Europe often with the participation of MB dealers.
If you want to know to what degree your vehicle was modified, check for AMG manufacturer's plate on the engine block and elsewhere and AMG part numbers on the suspension parts.
So, what is an AMG? Anything you want as long as the pieces are genuine.
"Lead, follow, or better yet, get out of the way!"E500 owners motto