There is no universal rating system, if that is what you are asking. IMBA does have one, but it is not used consistently. Many areas use a relative rating system similar to what ski areas do. This means that the green trails are the easiest ones there, and the black ones are the hardest ones there, but it is as they related to each other.

Like what the other posters say, it's pretty relative to who has made the determinations. I am currently working on a project where we are inventorying trails and designing a map, and it's a real challenge to decided how to rate something. Do we rate it for a visitor to the area, or do we rate it for the locals?

Most rating systems take the following into consideration: aerobic effort, technical difficulty, and risk.

Some areas don't have true "beginner/easy" stuff. We have one riding area that does have a lot of almost flat, buff trails with little to no obstacles which is great for beginners. But the other area ( the one I am mapping) has NO flat trails, and obstacles on everything ranging from slickrock to gap jumps.

Even on your easiest rated trails, IMSHO as a mountain biker there should always be something you can't ride. That's why you keep going back, to try it again and again until you "get it" and have a YAHOO moment.