At the age of 29, Siddhartha Guatama was deeply troubled by the suffering he saw around him, and he renounced his privileged life to seek understanding. After 6 years of struggling as an ascetic, he finally achieved Enlightenment at age 35. After this, he was known as the Buddha ("One who is awake"). After all of these experiences, Guatama realized that everything is subject to change and that suffering and discontentment are the result of the attachment to circumstances and things which, by their nature, are not permanent. From that point on, the teachings of Zen Buddhism have been passed down from teacher to students. Around 475 AD, Bodhidharma traveled from India to China and introduced teachings there.
In general, Zen is different from other religious groups. Zen is not a religion in the sense that religion is generally understood. Zen has no God to worship, no ceremonial rights to observe, no "future abode" to which the dead are destined. Zen has no set doctrines which are imposed on its followers for acceptance. Zen teachings come out of one's own mind. It's followers claim it to be a living experience, a "creative impulse."
All major religions, Buddhism included, have split into schools and sects. But the different sects of Buddhism have never gone to war with each other and they go to each others temples and worship together. The types of Buddhism all may seem very different but at the center of all of them is the so-called 'Four Noble Truths' and 'The Eightfold Path'
The Four Noble Truths
The first truth is the observation that suffering or unhappiness, referred to as dukkha, is pervasive in life. Dukka is explained to be suffering or unhappiness of any kind. (i.e. the desire for wealth or respect, the distaste for bad weather).
The second truth explains that the cause of dukkha is craving or clutching at life. Our unhappiness results from our desiring to make life fit our preconceptions of what should be or what we would like it to be.
The third truth explains that dukkha can be ended by ending the craving, which in turn, can be achieved by following the fourth truth.
The fourth truth reveals to follow The Noble Eightfold Path.
The Noble Path
The first and second relate to right views and right understanding of the mind. These proposals require proper understanding of Buddha's method (nature of dukkha).
The third, fourth, and fifth paths refer to right speech, right conduct, and right vocation. They offer simple suggestions of prudence. One should follow "the path" to achieve spiritual goodness.
The sixth, seventh and eighth paths apply to meditation. Right effort, right awareness(smiriti), and right contemplation (smadhi) are necessary to achieve complete meditation.
Zen in Japan
As in china, after a while the Japanese government became fearful of the power Buddhist Temples had over the people. Eventually they ordered the samurai to destroy many of the temples. Ironically, this made Buddhism more powerful. The samurai were warriors in dangerous times. They were duty-bound to hold their lives cheap, risking death constantly. This was as hard for them emotionally as for anyone else. They had assumed the Zen monks would run screaming at the sight of violence, but instead, many of them were unintimidated. Some of them meditated inside their temple even as the fires consumed them. This impressed the samurai greatly and they began to study Zen.
Around that time a samurai called Musashi was making a name for himself. He was literally unbeatable, once killing over 30 men armed with guns, swords and arrows using only his famous two swords. Towards the end on his life he wrote a book, The Book of Five Rings, summarising his unorthodox technique. The book draws heavily on Tibetan and Zen influences to explain his ideas. It starts with Musashi bowing to Kanon (Avalotitesvara). The name itself comes from the Buddhist Theory of the five elements, Earth , Water, Fire, Wind and Emptiness/Void. These are reflected in the Tibetan style stupas that were popular in Japan at the time. The name of these stupas, Go Rin To (Tower of Five Rings), is almost the same as the name of the book Go Rin Sho (Book of Five Rings).
Zen also helped that samurai sharpen their skills be making their technique more spontaneous.
It later even split the major martial arts schools into those with a philosophical element (Judo, Kendo, Karate-do) and those purely about attacking people (Jujutsu, Kenjutsu, ending in "jutsu" which means technique).