The planeswalkers are an innovation of Wizards of the coast. It combines the aspects of the creature types and the enchantment types but cannot be targetted by spells or effects that target either premanent type. They can be killed by cards that destroys a target permanent, by its owner using up its loyalty counters or by attacking it directly with your creatures.
Ajani is quite a powerful card. It combines the power of three earlier cards. The first is the Icy Manipulator which taps any permanent in sight. The second is the Lightning Helix which dealsthree damage to a target and nets you three life. The third is Armageddon which destroys all lands. However, this version is special since you can choose to smash all the opponent's lands without smashing yours.
I have fought with this card and fought against it as well. It is simply amazing and hard to kill. This version is a foil prerelease. At $6.00 it is a steal.