Links:
Climbing FAQ at TradGirl

Ratings:
Class - The Yosemite Decimal System (YDS)

The Yosemite Decimal System (YDS) of grading routes was initially developed as the Sierra Club grading system in the 1930s to rate hikes and climbs in the Sierra Nevada range. The rock climbing portion was developed at Tahquitz Rock in southern California by members of the Rock Climbing Section of the Angeles Chapter of the Sierra Club in the 1950s.[2] It quickly spread to Canada and the rest of the Americas.

A YDS rating consists of 3 parts: the Grade, the Class, and the protection rating. The Grade and protection rating are optional, and usage varies widely. When the climb also involves aid climbing, the aid climbing grade can be appended to the free climbing rating. Some examples: El Capitan - The North America Wall VI, 5.8, A5

The grade has to do with the length and seriousness of the route, ranging from "Grade I - one to two hours of climbing" to "Grade VII - a climb lasting a week or longer".

The class of a route is derived from its "crux" or hardest move. If you hike a class 2 trail from point A to point B, and are required to scale a 20 foot wall of rock (class 4) along the way, then you hiked a class 4 trail regardless of the normal exposure.

RJ Secor quips:

• Class 1: you fall, you're stupid.
• Class 2: you fall, you break your arm.
• Class 3: you fall, you break your leg.
• Class 4: you fall, you are almost dead (i.e., you can't breath and move your arms, legs, and head).
• Class 5: you fall, you are dead.

class 1 Hiking on trail (used for some peaks that do NOT have trails)

• walking along a clear, well established trail (sidewalk)

class 2 - Scrambling with occasional use of hands

• cross country, requiring route finding skills
• thick brush, climbing over and around fallen trees, and big talus - hands are used for balance.

class 3 Steeper climbing with handholds, some exposure, usually no need for a rope.

• MUST use your hands for progress but don't need to search for holds nor do you need Real Rock Climbing techniques

class 4 Steep, exposed, but easy climbing with rope for safety.

• use a rope to avoid serious injury on a fall, but don't place protection.

class 5 Technical rock climbing (Free Climbing) - specific climbing moves necessary, with a rope and intermediate protection necessary.

• climbing involving technical moves and protective hardware in case of a fall
• thin, exposed climbing, requiring skill (the holds are not obvious to a novice - this is where weird moves such as laybacks, underclings, and evangelical hammerlocks come into play_
• Real Rock Climbing(TM) where most people will use a rope (but where some very experienced people won't feel the need) and where serious injury or death is very likely if you take an unprotected fall

5.10 and higher are broken down into sub-categories a,b & c

class 6 Old Aid climbing. (not actually part of the YDS)

• The rope bears the climber's weight on purpose
• Rope ladder (
• Use equipment for aid

• A0 or C0: Aid points are fixed
• A1 or C1: Aid placements are solid and easily placed.
• A2 or C2: Placements are awkward to place and hold less.
• A3 or C3: Aid placements will hold a short fall.
• A4 or C4: Aid placements only hold body weight.
• A5 or C5: Entails enough A4 placements to risk a substantial fall.

• G - Good, solid protection ground up.
• PG - Pretty good, few sections of poor or non-existent placements.
• R - Runout, some protection placements may be very far apart.
• X - Little or no trustworthy protection, extremely dangerous.

• Grade I - easy excursion. From a single pitch up to a few hours. Not particularly committing.
• Grade II - Bit more serious, but less than a half day climb. You could get a few II's in in a day if you worked at it.
• Grade III - Half day climb. (or thereabouts)
• Grade IV - A full day
• Grade V - A day or two. Moonlight Buttress in Zion.
• Grade VI - A few days. The Nose is a grade VI climb.
• Grade VII - Expedition - Bring a gallon of tequilla, you'll need it.

The British rating system - The Brits have two different ratings (they say grades): an adjectival grading and a technical grading. The adjectival grading says something about the overall difficulty of the climb. The technical grading says something about the most difficult move of the climb. So, an easy climb with a difficult move would have a relatively low adjectival grading with a high technical rating (like E4/6c).

Easy (5.2)
Moderate (M)
Difficult (D) (5.4)
Very Difficult (VD) (5.5)
4a	Severe (S)	Hard Severe (HS)
4b			Very Severe (VS)
4c	(5.8)			Hard Very Severe (HVS)
5a	(5.9)				Extremely Severe (E1)
5b	(5.10a-b)					E2
							E3	E4
5c	(5.10c - 5.11a)								E5
6a	(5.11a - 5.11d)
6b	(5.11d - 5.12c)									E6 - E9
6c	(5.12c - 5.13a)
7a	(5.13b - 5.13d)
7b	(5.14)

More Rating/Grading Systems:
The Union Internationale des Associations d'Alpinisme (UIAA) rating scale, an ill-fated attempt at international standardazation, is used mostly in Western Germany and Austria.
It used to be a scale ranging from 1 (easy) to 10 (very difficult), but has been extended to 11+/12-

The Angeles Sierra Club Chapter system on the hike ratings page.

Rock Climbing Rating Table (UIAA, French, British, Australian)
Comparison chart for different free climbing rating systems. (UIAA, YDS, UK, French, Saxon, ...)
Ari's Base Camp: Grading/Alipne

Glossary/Terms:

• Aid Climbing: the technique of using gear to support your weight as you climb. As simple as using a bolt as a single hold, or as complex as climbing an entire route with full weight on your gear.
• Belay: a system of setting up the rope to hold a climber in the event of a fall. A procedure that manages the rope by taking in or letting out slack to minimize the seriousness of a fall. Usually performed by a second person with a belay device to provide friction on the rope. The belayer may stand freely below the climber in the case of top-roping or be fastened to an anchor point to support a leader fall or when belaying from above.
• Chalk: Magnesium carbonate gymnastic chalk used to absorb sweat, resulting in increased adhesion to holds.
• Chock: A stone or object that becomes wedged in a constriction point in a crack, or more commonly for climbers: passive wired protection devices, called nuts, stoppers, wires, hexes, etc.
• Clean: A) climbing without the use of pitons, bolts or anything requiring a hammer. B)Climbing without falling or hanging. C) removing equipment from a climb.
• Free Climbing: the technique used when ascending a pitch using only hands, feet and body English, while placing gear for protection only.
• Free solo: Climbing without using ropes for protection. A fall usually implies very serious injury or death.
• HMS Carabiner: Halbmastwurfsicherung carabiner. Pear-shaped carabiner which due to its wide top is great for belaying from and setting up belays on.
• Lead climbing occurs when the climber (and the Rope) start at the base of the climb. Protection is placed periodically throughout the climb to reduce the distance that the climber can fall.
• Protection: A point of security such as a chock, piton, bolt or other device where climbers can clip in.
• Sport climbing: Climbing well bolted or pre-protected routes. Sport climbing often implies difficult to extreme single pitch face climbing.
• Soloing: Climbing alone, usually without protection.
• Trad: Traditional climbing, characterized by the placing of protection (cams, nuts, etc.) in cracks and pockets. Trad also includes multi-pitch routes often with long runouts.

Safety:
Fifty-one climbers died in Yosemite from traumatic injuries between 1970 and 1990. THere are at least 50 fractures per year), and a much larger number of cuts, bruises, and sprains.
Most Yosemite victims are experienced climbers, 60% have been climbing for three years or more, lead at least 5.10, are in good condition, and climb frequently. Short climbs and big walls, easy routes and desperate ones: all get their share of the accidents.
Source: Friends of Yosemite Search and Rescue -- Climbing Safety
one-third of serious leader fall injuries were because they did not place enough protection and two thirds were from protection failure under the force of the fall.
Source: Yosemite NPS - Climb Safely
Anchors in Earnest - Basic Anchor Considerations for Experienced Trip Leaders, by Cyril Shokoples

See also:
Types of rock climbing at Wikipedia

Links:
Climbing Gear
Climbing Rescue
Rock Climbing in Yosemite
American Mountain Guide Association (AMGA)
Climbing Destinations
abc-of-rockclimbing.com
Climbing at about.com
How To's at Bay Area Climbers