What is autocross .....
Autocross (also known as auto-x or Solo events) is an all forward motion driving skill contest. Each driver is individually timed to the hundredth of a second over a short, miniature road course clearly defined using traffic cones. Cars compete one at a time against the clock, hence the name "Solo", with elapsed time and appropriate penalties for course deviations being the determining factor for awards. Autocross does not preclude the running of more than one car at a time, provided they are separated on course by adequate time and distance to eliminate any possibility of a passing situation. An event can be held on any relatively flat paved surface, usually a parking lot, or airport apron or runway.

Autocross emphasizes driver skill and vehicle handling rather than just speed. The corners are tight, and there are lots of them, so the driving is exciting and challenging. Special competition licenses are not required, and hazards to spectators, participants and property do not exceed those encountered in normal, legal highway driving. The skills you learn and practice here; smooth transitions, enhanced braking, and skid correction, will have an immediate impact on improving the safety and skill of your street driving. Autocross is an excellent way to teach car control to young drivers in a safe environment.

Autocross is also a very social sport, filled with some of the friendliest people you'll ever meet. The camaraderie of the drivers is a special part of autocrossing that is profoundly satisfying. The costs of autocross competition are reasonable and most people participate with their every day use automobiles. Entry fees are usually no more than $35 per driver, and two or more drivers can share the same car. A day of autocrossing is far safer for both car and driver than most people's daily commute to work.

More information can be found:
http://www.capitaldrivingclub.com/faq.html#Whatisautocross
http://www.tirerack.com/features/solo2/handbook.htm