What is the difference between dirt bikes?

The manufacturers break their dirt bikes into two categories, motocross and off-road. Within the off-road category you have the liquid-cooled high performance bikes which are for woods racing (or hard core trail riding) and the air-cooled low performance "trail bikes". In Honda's lineup for instance you have a CRF250R which is a motocross bike, the CRF250X which is a high performance liquid-cooled woods bike and the CRF230F which is an air-cooled low performance trail bike. The high performance woods bikes are very similar to the motocross bikes. They share the same frame, suspension, engine, etc. but are tuned a little differently (engine and suspension) for woods riding. The air-cooled trail bikes really share little or nothing in common with their motocross siblings.