Zen is an East Asian form of Mahayana Buddhism that focuses on regular seated meditation practices that for beginners include momentary awareness of the "now" or "present moment". It started in China during the Tang dynasty with an itinerant monk from India named Bodhidharma. It them spread along with the Asian martial arts to Japan, Korea and Vietnam.
One of the best descriptions of what Zen is can be understood when the you understand the saying: "Before enlightenment, chop wood, carry water. After enlightenment, chop wood, carry water."
Buddhism began with Shakyamuni Buddha, the founder, about 500 BC in Northern India. Zen primarily emphasizes meditation to realize the illusory nature (meaning impermanent, interdependent "merely labelled" or "imputed") of the self and the eternal abiding reality of the "Cosmic Buddha" or 'Dharmakaya'.
Buddhism moved from India to China due to the efforts of the monk Bodhidaruma (or 'Bodhidharma') who made the treacherous sea crossing to China. The Indian word for meditation 'Dhyana' was taken to describe the heart of the practice in China and the name 'Dhyana' was there pronounced 'Chan'. When Chan Buddhism was taken from China to Japan the word 'Chan' was pronounced 'Zen' in Japanese. There are several schools of Zen Buddhism in Japan: Soto, Rinzai, Obaku etc. Another name for the meditation which is the heart of the practice is 'Shikan Taza' which means 'Serene Reflection' - a "seeing deeply into the nature of things".
Chan Buddhism is a form of Buddhism which originated in China in around the 6th Century A.D. Today it is notably practiced in China (as Chan Buddhism), Japan (as Zen Buddhism), Vietnam (as Thien Buddhism) and across Korea (as Seon Buddhism). Many liberals and "beginner Buddhists" mistakenly consider Zen as a philosophy rather than a religion. Other liberals and beginner Buddhists, while considering Zen as a religion, mistakenly identify it as a non-theistic religion.
Across Zen's various schools, the emphasis of Chan Buddhism is on regular meditation (known as zazen or shin-ja) and heightened awareness of 'things in themselves' through various techniques, including through the focus on repetitious daily activities (as is common in various Christian monastic communities) including chanting and prayer.