The Yosemite Decimal System (YDS) consists of five general classes, the fifth being subdivided with a decimal notation, but it is generally accepted that some ratings are too high or too low because people want to brag or because the standards among climbers has changed over time, and many people have editorial comments about the YDS.
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.

The argument that exposure has nothing to do with the class of the climb is not supported by the history of YDS. Exposure was implied in the definition of 4th and 5th class by the use of a rope. Starting in the 5th edition of Freedom, exposure was included in the class descriptions, probably because of the wide variety of techniques being used on the same climbs. In the 6th edition of Freedom, the wording has changed again.
Here are the five classes, where each bullet represents a different opinion as to what the class actually represents. When you are reading a route description or trip report, keep in mind that there may not be general agreement on ratings. The astonishing thing about the email discussion which prompted this expanded definition is that almost everyone thought that almost everyone ELSE agreed with them!

class 1 (used for some peaks that do NOT have trails)
· hiking on trail
· bike path
· walking, preferably uphill
· walking along a clear, well established trail (sidewalk)

class 2
· cross country, requiring route finding skills
· cross-country, using hands for balance
· hiking trail (goes with class 1 is a bike path)
· difficult cross-country travel (thick brush, climbing over and around fallen trees, and big talus - hands are used for balance)
· the trail is either non-existent or very uneven or intermittent and you may need to put your hand down once in a while for balance.

class 3
· scrambling on rocks using hands as well as feet
· requires use of hands for climbing, rope may be used
· I need my hands but might survive a fall
· hands? Maybe. (goes with class 1 is a bike path)
· rope is necessary only to provide comfort
· MUST use your hands for progress but don't need to search for holds nor do you need Real Rock Climbing(TM) techniques.

class 4
· climbing on steep terrain requiring roped belay
· I would die if I fell
· hands? Yes! (goes with class 1 is a bike path)
· exposed climbing such as a ladder going up the side of a water tank (belays should be used)
· use a rope, but don't place protection
· rope required to prevent serious injury if a fall occurs
· why don't we just ditch Class 4 and call everything 5.0 that used to be Class 4!
· you are leading along and it is not too hard and when you get to the end of the lead you notice that you haven't felt motivated to place any protection

class 5
· climbing involving technical moves and protective hardware in case of a fall
· safety rope (goes with class 1 is a bike path)
· 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_
· leader places protection along the way
· 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

"The experienced climber, having accomplished or attempted free climbs of varying degrees of difficulty in the YDS class 5 range, gains an understanding of the level of difficulty involved. To the beginner, however, these ratings are simply a set of numbers, understandably, easy if rated 5.0 and impossible if rated 5.13. To provide a slightly better understanding within the class for the beginner the following tongue-in-cheek description is provided:

5.0 to 5.4
There are two hand- and two footholds for every move; the holds become progressively smaller as the number increases.

5.5 to 5.6
The two hand- and two footholds are there, obvious to the experienced, but not necessarily so to the beginner.

5.7
The move is missing one hand- or foothold.

5.8
The move is missing two holds of the four, or missing only one but is very strenuous.

5.9
The move has only one reasonable hold which may be for either a foot or a hand.

5.10
No hand- or footholds. The choices are to pretend a hold is there, pray a lot, or go home.

5.11
After thorough inspection you conclude this move is obviously impossible; however, occasionally someone actually accomplishes it. Since there is nothing for a handhold, grab it with both hands.

5.12
The surface is as smooth as glass and vertical.

5.13
This is identical to 5.12 except it is located under overhanging rock."

It can sometimes be helpful to know who rated the climb; since ratings are give by humans and not by computers, human frailties can be interjected into the system. For example, some climbers will intentionally underrate climbs so that those who follow later will be impressed. Fortunately most climbers do attempt to give accurate ratings, and by the time a route is in a guide book, enough people have made the climb to show the rating as a consensus of opinion. Also remember that ratings are given for ideal conditions; unfavorable conditions can cause considerable change in a climb's difficulty.

The increasing technical difficulty of Class 5 climbs led to the same "relative grading" problem that had caused the initial development of the system, so that class was subdivided in the late 1950s. It was developed by Don Wilson, Royal Robbins, and Chuck Wilts in 1956. Initially it was based on ten climbs in Tahquitz, California, and ran from "The Trough" at 5.0, a relatively modest technical climb, to "The Open Book" at 5.9, considered at the time the most difficult unaided climb humanly possible. However, advances in techniques and equipment have since led to harder climbs being completed. The first such climb was given the rating 5.10; the second the rating 5.11. It was later determined that the 5.11 climb was much harder than 5.10, leaving many climbs of varying difficulty bunched up at 5.10. To solve this, the scale has been further subdivided above the 5.9 mark with a-d suffixes. It is now an open-ended scale, with 5.15a the hardest climb having been completed (as of October 2003). In addition, the popularity of bouldering (very short unroped extremely hard climbs) introduced the current standard "V" scale, which currently ranges from V0-V15. Boulder problems tend to be short and powerful, often requiring different techniques than roped climbing, and a separate rating system makes sense.

Danger ratings:

However, all the various rating systems do not describe the danger level faced by the leader should the leader fall (which is one reason why the YDS should not be used in climbing gyms). Various rating systems were introduced to include some description of the potential falls. These ratings which describe danger and psychological difficulty are not nearly as refined as the ones describing physical difficulty. Currently, the most widely used is the R, R/X, and X ratings. These crudely describe the danger level.

R runout, where a fall would likely result in serious injury
R/X very runout, where a fall at the wrong place will likely result in at least serious injury and possibly death
X extremely runout, where a fall at the wrong place will likely result in death
Some guidebooks also use a PG and PG/R system, in some guidebooks PG means partway runout, and in others means "Protection Good."

These ratings do not describe other hazards such as loose rock, which often can only be negotiated safely with a decade of mountaineering experience, and which regularly kill even the best climbers in the world.