Zen is a form of Buddhism which especially emphasizes meditation. It developed when teachers from India, the original homeland of Buddhism, founded schools in China, where it was known as ch'an. The canonical exemplar of these teachers is Bodhidharma.
Possibly influenced by local Taoism, these schools de-emphasized study and worldly deeds, and concentrated instead on meditation and a non-rational awareness of the world and the way the mind reacts to it.
Later schools developed the famous koans, paradoxical "riddles" or "puzzles" designed to shock the mind out of its rationalistic rut and into a non-discriminatory awareness.
Many modern students have made the mistake of thinking that since much of Zen sounds like nonsense, then any clever nonsense is also Zen. This is not the case, but see Discordianism and the Church of the Subgenius for modern semiserious religions influenced by this idea.