whats the difference?

Your Standard Monster truck class are usualy noted if they have a Center Transmission and ride Heigth for the motor to the ground.

To clarify the unlimited class would include 'truggies' and the big block class would be limited to true monster trucks.

Ditto to what pat said. Revo, TNX would be examples of small-block class. LST, MGT, Savage would be examples of the big block class. Unlimited would include XTM Mammoth, GS SUT, and everything else.