I hadn't heard of Kramnik defending his title but here's some quick information to get things started.
Kramnik is the player who defeated Gary Kasparov in 2000 to end Kasparov's decade-and-a-half domination of the chess world.
Acording to the
official FIDE ratings, Kramnik is ranked #3 in the world. He's a decent amount behind Kasparov who many think is still the best player despite losing the World Championship. He's also a bit behind V. Anand who is widely regarded as being in the same league as both Kasparov and Kramnik (but certainly a bit behind Kasparov until he proves otherwise).
I don't know much about Leko other than that he's currently ranked fourth by FIDE and he's a rather young 24. He's never competed for the World Championship before.
You could try posing the question in the
Fide Forums.