Both the ATP and the WTA have two rankings: the traditional ranking, also called Entry Ranking, and the new Championships Race.

Explanation: basically, the Entry Ranking ranks the players considering the immediate past 52 weeks and is used for determining qualification for entry and seeding in all tournaments, while the Championships Race ranks the players in calendar-year basis and is useful to show who's "hot" the current year. Usually when you hear: "World number one Roger Federer", they are talking about the Entry Ranking...