What are the international rafting classifications?

CLASS I: Slow moving water with No significant waves. No obstacles.

CLASS II: Easy. Rapids with regular waves and some rocks with clear passages.

CLASS III: Moderately difficult, irregular waves, often with narrow channels and obstacles such as rocks and small hydraulics that should be avoided.  

CLASS IV: Difficult fast moving water Complex channels with drops and many significant obstacles to be avoided. Precise maneuvering is required.

CLASS V: Extremely difficult, long violent rapids, often flowing into each other almost without interruption. Class V includes sections with high consequences, and advanced, aggressive paddle strokes are needed for successful navigation.