Different tracks have different classes, and the amount of classes.
You will normally find at the least a "Sportsman" class, and some type of advanced class. Sportsman is normally a HUGE class, since It consist of anybody from a beginner, all the way to the next level (advanced), and includes some that "should" be in advanced, but like to win all the time (that is another thread all together!!
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Some tracks/race events break it down even further, and offer even more classes (beginner, novice, amateur, expert/pro). Always try to pick a class that you are going to feel comfortable in. Personally, I race a little over my head. It means the chances of me winning are less, but I feel it makes me a better driver, and the competition is higher. As long as I dont feel I am slowing others down, its all good. If I can run in the middle of the pack, it gives me something to work toward. If you find yourself in last all the time, and causing a scene out there, you may be in the wrong class.