71 ( +1 | -1 ) Kasparov, unhappy with the way FIDE was handling affairs, broke away from FIDE back in 1993 and tried to start his own chess federation. From then on, there were two "world champion" titles, one held by Kasparov (until he lost it to Kramnik) and the one handed out by FIDE. Hence the need for this year's world championship reunification cycle.
Opinions differ, but my personal feeling is that the title currently held by Kramnik holds more weight since he won it by not only defeating Kasparov, the world's #1 ranked player, but he did so under tournament match conditions. Contrast this with Ponomariov's title, which he won through an FIDE knockout tournament.