No problem mate were all here to learn,
The diffuser is the section with fins to channel air
Fits in between the exhaust tips and generally painted a different colour to the body of the car.
As for the grey part along the top of your bumper
You can remove and spray this but consider fitting an ST rear bumper prior to investing in the existing bumper.
Ahh, is there a way to remove my current lower bumper part and add a diffuser? Also cant seem to find a decent st bumper in black.. all need respraying and not gonna pay £200-£300 for a bumper.