Basically theres a few classes (based on whether it is street trim cars or race trim cars, and driveline ie FWD RWD or 4WD), which determine who you compete against.
From what I understand the main rules are that the tyres must be street legal (ie R compound semi slicks typically, no full slicks allowed), and i *think* the mounting points of the suspension must remain factory (ie no onboard dampers or whatever)
Dont think tube frame chassis or anything like that are allowed either.
Engine wise (in terms of HP restrictions etc), I think its basically unlimited, and its more the design of the track that keeps a lid on things (ie its quite tight and technical, so endless HP isnt the answer because the track is this way and the lack of traction without full slicks being used)
Correct me if Im wrong but Im pretty sure thats the gist of it