What determines categories in drag racing?

Basically, how fast your car is is what determines what class you run in, when it comes to bracket racing. They wikk have brackets from, say, 14.99 and slower, 12.50 to 13.99, 10.99, to 12.49, and so on. In class racing you build a car to compete in a certain class, by the rules governed by NHRA, in their official rule book.