Class I (Easy). Is the water flowing with a few small waves and perhaps some rocks off to the side? If so, you're probably in a Class I rapid. These are not the rapids that build a guide's repertoire of campfire bravado. Note their presence, but relax and enjoy the scenery.
Class II (Novice). Like Class I rapids, Class II rapids rarely are named. Waves may be up to approximately three feet high, so you may get a little wet. Get used to it.
Class III (Intermediate). Now is the time to sound the barbaric yawps. Just as you're getting all giggley and gooey like with your guide and fellow rafters, you'll hear the first Class III coming and look up to see perhaps a small falls, some large rocks and certainly some large waves. Intermediate boaters will have no problem guiding through these rapids, but due attention should be paid.
Class IV (Advanced). Take these rapids very seriously, stab hard with your paddle, and you'll be shouting excitedly and wearing a big smile when you reach the still water below. Class IV rapids are marked by turbulent waves, a hard current and rocks forcing skilled navigation.
Class V (Expert). Class V rapids are hardcore whitewater. Many commercial rafting trips don't include these rapids, but even beginning rafters can experience a Class V with an expert guide. Careful scouting, constant vigilance and expert navigation are required to avoid large rocks and to precisely negotiate deceitful currents, steep drops and colossal waves.
Class VI (Extreme). Once labeled as unrunnable, Class VI rapids have been negotiated by teams of experts. Class VI rapids are where you speak to the gods, whispering your own quiet prayer. Fierce water pounds through chaotic chutes marked by many forbidding rocks and stomach churning drops. Only experts are allowed to put themselves in the throes of a Class VI. Commercial rafting does not occur on Class VI rapids.