Rating
Rating shouldn't be aliased to rank, in the context of Go. However sometimes people will use the former in determining the latter.
A rating is a calculated measure of playing strength. The most commonly used rating in Europe is GoR. However other formulae exist, on yahoo and in
AGA for example.
One starting point for studying rating algorithms is Elo Rating.
Another less traditional example is the KGS Algorithm, although one could argue this is producing Ranks not Rating.
See also: Rating Systems