It is said that Zen cannot be described by words and that it is something that needs to be experienced. Thus, a full description is beyond the scope of this reference. A close approximation would be Enlightenment. Zen is a form of Mahayana Buddhism, specifically a Japanese form of the Chinese Ch'an tradition ("Ch'an"comes from Sanskrit "dhyana", or meditation). Zen's origins are traditionally traced to Bodhidharma (dates unknown, probably d. circa 530), and from him to Mahakasyapa, one of the Buddha's disciples.
Within Zen, there are many schools that develop awareness and enlightenment using many different techniques. The two major schools of Zen are Soto and Rinzai. Soto tends to stress zazen meditation, while Rinzai tends to focus on koan practice. The goal is to break down the illusionary and conflicting nature of the material world. Zen followers develop a form of awareness in which every experience is as fresh as the first. At heart of Zen lies the concept of sudden realization.
Zen and Zen Buddhism are one and the same thing.
References:
Heinrich Dumoulin, History of Zen Buddhism; Sohaku Ogata, Zen for the
West (1959 & 1973); Nancy W. Ross, The World of Zen (1960); Alan
Watts, The Spirit of Zen (1958); P. Yampolsky tr., The Platform Sutra
of the Sixth Patriarch (1971); D.T. Suzuki, Essays in Zen Buddhism
(three series, all London: Rider, 1970); D.T. Suzuki, Introduction to
Zen Buddhism (London: Rider, 1969).