On this website we have decided to use a similar grading system to that used in the UK by Forest Enterprise. Its slightly different to the IMBA grades used in Australia where they have only one grade (Blue) covering both the moderate (Blue) and difficult (Red) that we prefer. We feel it gives a better indication for people relatively new to the sport.
As with all grading systems its VERY subjective and is only an indication. Ride within your limits and ride what you can see. (This is good advice but we would be lying if we said we always follow it!!)