The man known as the Buddha lived around 550 B.C. in India and before he died he started a religion whose impact on Asia cannot be measured. Although it eventually died out in its native India, Buddhism spread to Nepal, Tibet, China, Korea, and Japan, as well as the countries of South East Asia. Buddhism was already over a thousand years old when it reached Japan and had changed considerably in those thousand years.
The Buddha was concerned with just one thing - how to end suffering. Indians back then, like many today, believed that all living things are reborn in a constant cycle of birth and death. The Buddha also believed this and concluded that if we could break free from this cycle, we could end the suffering that goes with living. His Four Noble Truths sum it up better than I can:
The Eight Fold Path describes the proper way to live to achieve enlightenment. It is not an easy path, and in theory it could take you several lifetimes to finally transcend the cycle of birth and death. The path demands great sacrifice and discipline. Obviously such a seemingly pessimistic and difficult religion is going to have some public relations problems. Joe (and Jane) Layman doesn't have enough spare time to spend hours sitting on his butt meditating. Neither are most people real interested in giving up married life. So why has Buddhism been so popular? The answer is simple: in Tibet and China it mixed with local shamanistic ideas and practices to become a ``Big Vehicle'' offering rituals and prayers to comfort the common people and offer them some hope of salvation in this lifetime. The Buddha himself was deified. Eventually there were a multitude of schools (sects) in East Asia each stressing some element of the Buddha's teachings or those of popular priests after him. In Southeast Asia Buddhism was not exposed to Tibetan or Chinese practices and so has remained much closer to original Buddhism. The Buddhism which came into Japan was of the ``Big Vehicle'' sort. Each class found a school of Buddhism that suited its outlook and station. Thus, the imperial court was drawn to sects heavy in ritual and philosophy. Commoners generally went for the simpler sects which promised them salvation. The samurai found Zen Buddhism perfectly suited to their needs - the need to die at anytime without any hesitation.