Autocross is a form of motorsports that emphasizes safe, low-cost competition and active participation. An autocross is a timed competition where drivers navigate one at a time through a temporary course marked by traffic cones, rather than racing on a track with multiple other cars, as in road racing or oval racing. Autocross tends to place more emphasis on car handling and driver skill than on sheer horsepower, and events typically have many classes which allow almost any vehicle, from economy sedans to purpose-built vehicles, to compete. Speeds are slower in absolute terms when compared to other forms of motorsports, usually not exceeding highway speeds, but the activity level (measured in driver inputs per second) can be higher than even Formula One due to the large number of elements packed into each course. In addition to being a national-level motorsport in its own right, autocrossing is a great way to get started in road racing.

Autocross events are usually held in large paved areas like parking lots or airfields. Typically, new courses are created for each event so drivers must learn a new course each time they compete. Prior to driving, a competitor will walk the course, taking mental notes, and developing a strategy to be refined upon subsequent runs.

The primary attraction of autocross is that it is a relatively inexpensive way to get involved in road-course-style motorsports. Because the lower average speeds, lack of physical obstacles and lack of wheel-to-wheel racing means that the potential for car damage, particularly from collisions, is very low, most autocrossers participate using vehicles based on production, road-going vehicles. It is not at all unusual to see an autocrosser race using his or her "daily driver." Many clubs use this aspect of the sport to try to attract new members by featuring classes for novice drivers.

The SCCA has ladies classes for autocross which share the same rules as the open classes but limit participation to women. It is common for husbands and wives, boyfriends and girlfriends, brothers and sisters or even two unrelated drivers to share the same car, but run it in their car's open class and its corresponding ladies class. The SCCA ensures that the ladies class for any given class is never run at the same time as the matching open class, specifically to allow car sharing in this manner. Women are not, however, prohibited from running in the open classes if they desire and many do so quite successfully.

Classes and rules vary from sanctioning body to sanctioning body, but typically the majority run lightly modified or unmodified (stock) vehicles. The most significantly altered production vehicles are the Street Modified cars, which retain production-based chassis but little else that is stock. Street Modified cars often produce in excess of 350 WHP, can reach 60 mph from a standing start in less than 3.2 seconds, and can corner in excess of 1.7 transient lateral Gs.

There are usually classes for purpose-built race cars imported from other series (including Formula Fords, Formula Atlantics, Formula 500s and vehicles similar to American oval-track stock cars) but most autocross cars are based on production cars.

The very fastest autocross cars are purpose-built "specials" (Modifieds in SCCA parlance) that feature huge, multi-element wings, snowmobile engines, and CVTs. While their top speeds are typically limited by gearing and the enormous aerodynamic drag from the huge wings, their transient cornering capabilities are unparalleled in motorsport.

Related autocross references: 

http://en.wikipedia.org/wiki/Autocross (Autocross Defined)

http://www.tirerack.com/features/solo2/handbook.htm (Novice Handbook)

http://www.sfrscca.org/solo2/faq/ (Frequently asked Autocross questions)

http://www.soloracer.com/articles.html (Several good articles and checklists)