Machines are classified for racing according to chassis style, displacement, and nature of modifications.  In general, classes are defined to maximize competitiveness and maintain close racing.  Some classes have lower-cost entry points, and more or less forgiving characteristics than others.  Durability, cost of maintenance, and rider skill are all factors in the selection of bikes and classes.  The following list is a high-level description of each class.  Full descriptions are available in the Rule Book and the PSRA is happy to explain the differences!