Without being more specific, its hard to say:
Cruiser: weighs 800lbs or more
Sportbike: weighs 375lbs or more
Cruiser: V-twin engine (2 cylinder)
Sportbike: Inline 4 engine (4-cylinder)
Cruiser: makes more power/torque at low RPM
Sportbike: makes more power/torque when rev'ed really high. Like 10,000 to 15,000 rpms
Cruiser: usually air-cooled engine
Sportbike: usually liquid-cooled engine
Cruiser: sometimes have suspension in the rear, sometimes not (called a 'hardtail')
Sportbike: always has suspension
Cruiser: has 'naked' engine
Sportbike: has plastic or fiberglass body panels called fairings. Used to cover the engine, improve aerodynamics, and to direct and force air into the cooling system.