Zen is a form of Buddhism that focuses on a momentary awareness of the "now".
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. It emphasises meditation to realise the illusory nature of 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 practise 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 practise is 'Shikan Taza' which means 'Serene Relection' - a "seeing deeply into the nature of things".
Chan Buddhism is a form of Buddhism which originated in China in around the 6th Century AC. Today it is notably practiced in China (as Chan Buddhism), Japan (as Zen Buddhism), Vietnam (as Thien Buddhism) and across Korea (as Seon Buddhism). There may be debate as to whether it should be defined as a non-theistic religion or a philosophical paradigm, but across its 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).