A street car is what the rules define it as, for example in NPK (and street outlaws), a street car is defined as having a vin, steel roof and stock look (rear wing is limited to 6 inches as I recall). Reaper got busted for having a carbon fiber roof/B pillar as I recall a few seasons ago in street outlaws. There are minimum weights depending on engine and power adder being used and only one power adder can be used.
The devil is in the details and in this case the rules are everything. There are shops and individuals that build cars specifically to comply with the rules. Is Tom Bailey's 5 second car a street car? He drives it on the street and it is "street legal".