Here’s a video describing those “class” ratings you may see on discussion boards like summitpost.com. It’s the Welzenbach System.
Class 1 is basic hiking, on trail and off; class 5 is straight up the rock face with ropes, pitons and all that stuff the GoBlog boys know how to use right (whole point of rock climbing? The ability to look down on mere hikers from a higher altitude, natch).
Video from Jason Klass and backpackingvideos.com.