International Rating Scale for Whitewater Rivers: What does class 3 or 4 mean? This international rating system is used to measure river difficulty. Anything above class 4+ is extremely high risk, and uninsurable, so chances are your buddy who did a class 5 river was exaggerating ;-)  
Class 2/3+: Illecillewaet River

Class 1: Easy. Fast moving water with small waves. Few obstructions, all obvious & easily missed with little training. Risk to swimmers is slight; self-rescue is easy.

Class 2: Novice. Easy rapids with obvious wide, clear channels, no scouting. Occasional maneuvering required, but rocks & waves are easily missed by trained paddlers. Swimmers seldom injured & group assistance seldom needed. Rapids at upper end of rating are Class 2+.

Class 3: Intermediate. Rapids with moderate, irregular waves; may be difficult to avoid, can swamp an open canoe. Complex maneuvers in fast current & good boat control in tight passages or around ledges required; large waves or strainers are present but easily avoided. Strong eddies & powerful current effects. Inexperienced should scout. Injuries to swimmers rare; self-rescue possible but group assistance may be required to avoid long swims. Rapids at lower/upper end are Class 3- or Class 3+.

Class 4: Advanced. Intense, powerful but predictable rapids requiring precise boat handling in turbulent water. May be large, unavoidable waves & holes or constricted passages demanding fast maneuvers under pressure. A fast, reliable eddy turn needed for maneuvers, to scout rapids, or rest. Rapids require must make moves above dangerous hazards. Scouting necessary first time. Risk of injury to swimmers moderate to high, & water conditions may make self-rescue difficult. Group assistance for rescue is often essential but requires practiced skills. Rapids at upper/lower end are Class 4- or Class 4+.

Class 5: Expert. Extremely long, obstructed, or very violent rapids which expose a paddler to added risk. Drops may contain large, unavoidable waves & holes or steep, congested chutes with complex, demanding routes. Rapids may continue for long distances between pools, demanding a high level of fitness. What eddies exist may be small, turbulent, or difficult to reach. At the high end of the scale, several of these factors may be combined. Scouting recommended but may be difficult. Swims are dangerous, and rescue is often difficult even for experts. Proper equipment, extensive experience, & practiced rescue skills are essential. Rapids at the upper end are classed as 5.1, 5.2, etc.

Class 6: Extreme and Exploratory. These runs have almost never been attempted and often exemplify the extremes of difficulty, unpredictability and danger. The consequences of errors are very severe and rescue may be impossible. For teams of experts only, at favourable water levels, after close personal inspection and taking all precautions. After a Class VI rapids has been run many times, it's rating may be changed to an appropriate Class 5.x rating.

Common Whitewater Terminology
Like any sport, paddling uses words that leaves those who aren't "river rats" scratching their hands in bewilderment. Here's some common words you may see in our website or hear our guides use. 

Boils: ascending currents that rise above surface level unpredictably.
Chute: a narrow constricted portion of the river.
Clean: free of obstructions; used to describe a route through a rapid.
Drop: a steep, sudden change in the level of the river bottom (6'+ drops called falls)
Eddy: a pocket of water downstream of an obstacle that flows upstream or back against the main current
Ferry: a maneuver used to move a raft back & forth across a river.
Gradient: the measurement of a river's descent in feet per mile or meters per kilometre
Hole: A swirling vortex of water where the river pours over an obstacle and drops toward the river bottom, leaving a pocket behind the obstacle into which an upstream surface current flows.
Hydraulics: a change in currents that causes surface features that can deflect, slow, or speed up a rafts descent (e.g., holes, waves, and eddies).
Keeper: a large hole or reversal that can keep and hold a raft or swimmer for a long period of time
Pool-drop: a type of river consisting of intermittent rapids followed by long, easy sections of calm water
Put-in: the place where a raft trip begins
Rapid: a place where the river leaves its two-dimensional state & enters a three-dimensional state with faster currents, rocks, & various types of liquid surface features
Scout: to walk along a bank to inspect the river
Section: a portion of river between two points
Strainer: an obstacle, such as a tree, that lets water flow freely through it but catches and entraps swimmers, rafts, & debris.
Take-out: the place where a raft trip ends
Technical: rapids containing many obstacles and requiring constant maneuvering
Throw bag: a bag that holds a long coiled rope, used as a rescue device to be tossed to swimmers.
Volume: the amount of water in a river

*these definitions as listed in The Complete Whitewater Rafter by Jeff Bennett, © 1996