Ponomariov is the player who won the Fide (Federation International de echec)world championship. However, back in 93(i think this is right), Kasporov, and the then challenger Nigel Short, broke away from Fide to play their match, forming a new federation of professional players.
As Kaspa didn't defend his title under Fide rules he was stripped of his Title, though, as all of the best players chose to play under the new federation (due to so called corruption on Fide's part) most people consider the Championship Kasporov and Short spawned to be the true championship of the World.