A 3-4 digit number signifying the relative skill levels of players. In each rated tournament all players will win or lose points based on a predetermined system infuenced by several factors. The most important factors are the final result and the strength of the opposition.
There are a few official ratings in the World today such as:
a) FIDE rating (or ELO rating) for international over the board chess. The highest ever FIDE rating was 2851, which Garry Kasparov had on the July 1999 and January 2000 lists.
b) ICCF rating for international correspondence chess.
c) National ratings for over the board chess and for correspondence chess.
All other ratings (including most of the ones offered by internet clubs) have just recreational value.