What class does my car fit in? Probably several. If you show up on street tires and do not know what class you want, you will be placed in one of the Street Prepared classes. Here is a brief description of the classes available. For more detailed rules please check out the SCCA Solo II Rulebook, available free at www.SCCA.com.
STOCK category has nine classes topped by Super Stock and then A through H Stock. SS is where those 'Vettes and Vipers go. HS is the home of the AMC Gremlin and the Geo Metro. In between come the Miatas and Mustangs, Porsches and Pontiacs, Nissans and Neons. It's not really the "nothing" category - there are things you can do, but they mostly involve replacing parts with better parts (tires, shocks, etc.) than actually modifying the car.
STREET PREPARED has six classes - ASP through FSP. More is allowed to be done to the car, but they are still basically (if barely) streetable and should be capable of being licensed for street use.
STREET TOURING is essentially a re-thinking of the next step up from stock. It goes a different direction from Street Prepared and probably falls between Stock and SP in its prep levels. STS is the primary class, permitting only sedan-type vehicles on DOT treadwear 140+ street tires, while STS2 is for two-seaters. STX ("eXtreme") and STU ("Ultra") is the same idea but allowing some additional cars and some other modifications. Salina Region has added a local class, STO ("other") which permits any car on street rubber to run. It is a great starter class.
STREET MODIFIED is an even newer category, aimed primarily at the new wave of modified imports that are becoming very popular. Engine swaps are allowed as long as the engine comes from the same manufacturer (a 6-cylinder Honda in a Civic, for example). There is SM and SM2 for different groups of cars.
PREPARED has been around as long as Stock, since the Solo program began. There are seven classes from A Prepared to G Prepared. These are full-out race cars built off production chassis. The concept originally began with Production- and Sedan-class road racers and most look like those type of cars. They're highly modified, run on racing slicks, and come in on trailers because there is no way they are street-legal. CP, which is the class for Mustangs, Camaros, etc., is one of the most popular classes in the sport.
MODIFIED also has been around from the beginning. There are eight classes, AM to FM, plus Formula SAE and Karts. AM, BM, CM and FM are mostly formula and sportsracer chassis, including everything from home built specials to full-out Formula Atlantics to wild-winged AMods that are almost whatever their builders can imagine. DM and EM are for wildly modified cars that began life as a production chassis. Formula SAE is the 600cc open-wheelers built to the collegiate F/SAE rules
KARTS also are technically in the Modified category but are their own animal. SCCA has Formula 125, which is basically any shifter kart up to 125cc or any clutch kart up to 100 cc or so. Karters have to wear leathers or skid suits because they are the only classes that do not require seatbelts. There also is a Junior Kart program which permits drivers age 8-15 to compete using 5 hp karts. Junior Kart participants must be SCCA members.