I'll have to double check my facts on this one, but off the top of my head as a quick sum-up...

Pure Land Buddhism centres on the Amida Buddha, an ancient Buddha who is reputed to have indefinitely delayed his own passing into nirvana in order to uplift the peoples of the world. He promised not to pass on into Buddhahood until he was able to bring everyone else along with him.

Of course, by committing such a sublime and selfless act it meant that he had already reached Buddhahood, but from that point forth it was believed that by invoking his name alone, one could reach singularity. This was a quantum shift from Buddhist belief at the time, which had for centuries advocated a life of reflection and meditation in order to pass on into the infinite and escape samsara, the wheel of reincarnation.

Needless to say, the Pure Land approach caught on quickly, as its demands were few and the promises were great...if you're ever near Chion-in you might find yourself muttering Namu Amida Butsu just in case it works.