Climbing Route Ratings

Routes are rated by the hardest move on the route. In the U.S., the Yosemite Decimal Rating System is most commonly used to classify climbing difficulty.

Climbing Route Classifications
Class 1	Walking an established flat, easy trail.
Class 2	Hiking a steep incline, scrambling, maybe using your hands.
Class 3	Climbing steep a hillside, moderate exposure, a rope may be carried but not used, and hands are used in climbing. A short fall could be possible.
Class 4	It is steeper yet, exposed and most people use a rope due to the potential of long falls.
Class 5	Climbing is technical and belayed roping with protection is required. It is not for a novice. Any fall from a Class 5 could be fatal.
Class 5 sub-categories
5.1-5.4		Easy	Climbing a steep section that has large hand and foot holds.
5.5-5.8		Intermediate	Small foot and handholds. Strength and rock climbing skills required. Low to vertical terrain.
5.9-5.10		Hard	Not for beginners. Technical, vertical and may have overhangs. Rock shoes required.
5.11-5.12		Hard to Difficult	Not for beginners. Technical, vertical and may have overhangs. Rock shoes required.
5.13-5.15		Very Difficult	Not for beginners. Technical, vertical and may have overhangs. Rock shoes required.

To further define a route's difficulty, a subclassification system of letters ( a, b, c or d ) is used for climbs 5.10 and higher. For instance, a route rated 5.10a is easier than one rated 5.10d. Some guidebooks use a plus (+) or minus (-) rating instead of the letters.

Source :

 http://www.rei.com/learn/expert-advice/getting-started-rock-climbing.html

http://en.wikipedia.org/wiki/Rock_climbing