I use this as a reference for the ABCD ratings
http://billiards.colostate.edu/threads/ratings.html#A-D
If you play in tournaments and leagues where scores are recorded, there is also the Fargo Rating which many places are migrating to using.
The number ratings are most often leagues and also local tournaments as the don't really have a solid reference and can vary from 1-5, 1-11, or whatever. A 5 in your home room based on a 1-7 scale would not be a 5 in another place that uses a 1-9 scale.
If you are wondering what your rating is, without knowing how you play but by the fact that you do not know anything about the rating systems, I'd say you would be a D player. Read the link and see what sounds like you. I just go by average balls you can make with an open table in 9 ball with ball in hand. If you can make 1-2 then make a mistake, that is a D. 2-4 or maybe 5 is a C, 5-7 is a B and 8-9 is an A.
You can also play the ghost which is break, take ball in hand and try to run out. If you miss, the ghost won, if you run out, you won. Play a race to 7. If you can beat the 9 ball ghost you are an A. A B would be able to beat the 7 ball ghost. A low C maybe the 3 or 4 ball ghost.