Class I- Easy:

Few or no obstructions; fast moving water with riffles; risk to swimmers is slight.

Class II- Novice:

Wide clear channels; occasional maneuvering; rocks and medium-sized waves are easily missed by trained paddlers; swimmers are seldom injured; assistance is helpful but rarely needed.

Class III- Intermediate:

Rapids with moderate, irregular waves which may be difficult to avoid and capable of swamping a canoe; complex maneuvers in fast current and narrow passages require good boat control; large waves, holes and strainers may be present but are easily avoided; powerful current; scouting advisable; swimmers are unlikely to be injured but assistance may be needed to avoid a long swim.

Class IV- Advanced:

Intense, powerful rapids; turbulent water; may involve long, unavoidable waves, holes or constricted passages demanding fast maneuvers under pressure; may require "must" moves above dangerous hazards; risk of injury to swimmers in moderate to high; group assistance to swimmers requires practiced skills.

Class V- Expert:

Long, obstructed, turbulent rapids; drops may contain very large unavoidable waves, holes or steep congested chutes with complex demanding routes; several of these factors may be combined at the upper level of this class; rescue is extremely difficult; above average rescue skills are essential.

Class VI- Almost Impossible:

Difficulties of Class V are carried to the limits of navigability; very dangerous; risks are high and rescue may be impossible; for teams of experts only under favorable conditions.

taken from: http://www.wildwaterrafting.com