There are many different rating/grading systems used to describe climbing and mountaineering routes. The YDS System is probably the most used locally. The Wikipedia Article on Climbing Grades is fairly exhaustive, covering the most commonly used ratings systems for rock climbing, ice climbing, mixed climbing, aid climbing, and overall route difficulty. It also includes a handy conversion table. A similar article can be found on SummitPost (including some examples of mountaineering routes of various difficulties).