The term Buddhism, used generically and rather loosely, is best understood as an ever-evolving phenomenon with three distinct aspects to its history:
The Heart Sutra, or the "Prajnaparamita Hridaya Sutra," to give it its proper Sanskrit name, belongs to the Buddhist tradition, and is probably the best known of the Mahayana sutras. It is chanted daily in the Buddhist monasteries of China, Japan, Korea, Tibet and in the West. This very short sutra (containing about fourteen verses in Sanskrit and 260 characters in Chinese) is a basic text of Zen tradition and is considered to contain the essence of all Mahayana wisdom schools.
Zen (Ch'an) began in China as a meditation school of Mahayana Buddhism and was partially shaped by its sutra literature. These sutras capture the dramatic fervor and religious aspirations of new movements in India that had broken away from the earliest forms of Buddhism (Hinayana), beginning, most likely, in the first century BCE. The Mahayana doctrine developed, religiously and philosophically, with the Bodhisattva ideal (which exhorted a practitioner to work for the liberation of all beings, however numberless, rather than striving just for one's own liberation) at its center, and the teaching of sunyatya (Emptiness) as its inspiration. D.T. Suzuki, the great facilitator between Zen tradition and the West, finds, in the psychology of the Bodhisattva, "one of the greatest achievements in the life of the spirit."