The Elo rating system is the generally accepted way of comparing the strengths of various players from around the world. These days there are six rating lists per year, so every two months the lists are modified to take into account recent results. Thousands of games from hundreds of tournaments are ELO-rated and the latest list gives an excellent indication of who is in form and who isn't!
Information on Elo rating changes is always important for leading players as many invitations are dependant on having a high enough rating. For chess players and fans everywhere it's the main measure of a player's form. World Champion Viswanathan Anand stays at the top of the May 2011 rating list.