Moderate trails are the ones that are going to be challenging with steeper hills, narrower trails, and harder obstacles. They are usually appropriate for intermediate to advanced users. Now that came from this web site: http://www.offroadnorthamerica.com/park.asp?pid=9
Now then having said that, I don't believe that there is any standardized trail rating formula. The best thing to do is when you come to a new trail that you may want to try, get out walk it, look it over, and then decide if you want to try it. Also, don't let anyone talk you into going onto a trail that you don't feel comfortable doing.