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. Many autocrossers compete in his or her "daily driver." 

TSCC uses SCCA classing; our classing guide is available here. There are different classes for different types of cars and different preparation levels, but the majority of TSCC members run lightly modified or unmodified (stock) vehicles, while some of our members compete in significantly altered production vehicles, and even full-up open wheeled race formula 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)