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Zen derives from the chinese word ch'an which in turn derives from the sanskrit word dhyana. Dhyana itself is a type or stage of meditation, the stages of which are dharana, dhyana and samadhi.
Ultimately, zen is a combination of doctrine and practises which seek to liberate the mind from the delusion of separation. Bhante Gunaratana has this to say in Mindfulness in Plain English
Zen meditation uses two separate tacks. The first is the direct plunge into awareness by sheer force of will. You sit down and you just sit, meaning that you toss our of your mind everything except pure awareness of sitting. This sounds very simple. It is not. (A brief trial will demonstrate just how difficult it really is.) The second Zen approach, used in the Rinzai school, is that of tricking the mind our of conscious thought and into pure awareness. This is done by giving a student an unsolvable riddle, which he must solve nonetheless, and by placing him in a horrendous training situation. Since he cannot escape from the pain of the situation, he must flee into a pure experience of the moment: there is nowhere else to go. Zen is tough. It is effective for many people, but it is really tough.
The story goes that in the 5th century CE, that Bodhidharma, a dharma master, and the founder of the Zen school arrived in India from China. More on that in Who is Bodhidharma, but in the meantime he reformed Buddhism in China and established the dhyana, a.k.a Zen school. He is call the first patriarch of Zen. However, it was Hui Neng, the sixth patriarch, that made it especially Chinese.