Grade 2: Some rough water, maybe some rocks, might require some maneuvering. (Skill level: basic paddling skill)
Grade 3: Whitewater, small waves, maybe a small drop, but no considerable danger. May require significant maneuvering. (Skill level: experienced paddling skills)
Grade 4: Whitewater, medium waves, maybe rocks, maybe a considerable drop, sharp maneuvers may be needed. (Skill level: whitewater experience)
Grade 5: Whitewater, large waves, large volume, possibility of large rocks and hazards, possibility of a large drop, requires precise maneuvering. (Skill level: advanced whitewater experience)
Grade 6: Class 6 rapids are considered to be so dangerous that they are effectively unnavigable on a reliably safe basis.