Classification of difficulty is necessarily somewhat subjective, and depends on flow levels, environmental conditions, and equipment used. However the following scale is widely accepted:
Class I: Moving water with small waves but no obstacles.
Class II: Bigger waves but no major obstructions.
Class III: Longer, tougher runs, considerably bigger hydraulics with waves, holes and stronger currents. Requires manouvering, but overall risk is minimal.
Class IV: Steeper, longer, and more heavily obstructed rapids requiring exertion and technical manouvering.
Class V: Very strong rapids with pour-overs, violent currents, big waves and boulders; major vertical drops and obstructions, and holes powerful enough to flip and hold rafts (and rafters). Carries some risks of injury and loss of equipment.
Class VI: Unrunable, with very significant risks; sometimes attempted by highly trained teams.
Class VII: Cannot be run.