Ever wonder what a Class 5.3 is? The American Safe Climbing Association has a great page with information on all the Rock Climbing Ratings used today including the Yosemite Decimals System orignally invented by Don Wilson, Royal Robbins, and Chuck Wilts in 1956 and the original Sierra Club rock climbing rating system, dating from the 1930s (below).
1st class – hiking
2nd class – scrambling and boulder hopping, hands are needed, but generally very little exposure or danger
3rd class – steep scrambling with exposure, ropes are needed for inexperienced people. An unroped fall on 3rd class terrain would likely be fatal.
4th class – steeper scrambling on small holds, ropes are needed for most people, but an experienced climber would normally climb an entire rope length without intermediate protection, then set an anchor and belay other climbers up. Inexperienced people may not be skilled enough to ascend even when belayed from above.
5th class – steep rock climbing where the leader must place intermediate protection, and in case of a fall, the intermediate protection would catch the leader (who will fall twice as far as the distance above the last piece of protection)
6th class – very steep climbing where the climber is unable to ascend the rock without pulling and stepping on rock protection