Reading the 14er guide books, you'll often see the difficulty of a climb or hike giving a rating or ranking of some type. Here are a few of the more commonly used rating systems, along with my own system that I devised for the site here (in case things aren't complicated enough!).

Yosemite Class System

The following are the different classes used in the Yosemite Decimal System.  Most peak baggers and guidebooks throw around these classes (sometimes also known in the US as "ratings" or "grades").

Class I Easy hiking.
Class II Hiking with some altitude gain and loss.
Class III Boulder hopping, occasionally using the hands.
Class IV Climbing, using the hands, but without the need for a belay (i.e. scrambling).
Class V Technical climbing with a belay (i.e. free climbing)
Class VI Aid climbing


Mike's Rating System

Although the Yosemite system is a good guideline for peak bagging, I have rated each climb with my own difficulty levels, based on my own personal gauge of the climb. I rate each climb on a scale from 1 to 5 (using these spiffy little mountain-esque buttons) :

 1- is a relative walk in the park...well defined trails, not too steep or exposed
2 - is a healthy hike. Trails may be harder to find and rougher, and may have a few steep sections and very minor exposure.
3 - will make you sweat and breathe hard. Probably requires off-trail route finding and boulder hopping. Trails can be hard to find and distance becomes an issue as well.
4 - requires hand-and-foot climbing. Some sections may be pretty steep and have some exposure, but it can be done without the benefit of ropes and tech gear.
5 - requires technical gear, lots of nerve and/or a death-wish