Zen Buddhism is a form of Mahayana Buddhism. It has as its main focus sitting, or zazen. Although Zen Buddhism does use sutras similar to the other sects of Buddhism, Zen is known for a non-verbal manner of transmitting the dharma and also for the use of Koans. Westerners are often confused about the purpose of the Koan, which often sounds like a riddle (What is the sound of one hand clapping?). The point of the Koan is not to get to an "answer", the point is to meditate on the question and arrive at enlightenment once the rational brain is exhausted by trying to "figure out" what is essentially un-figure-out-able.1
The non-verbal transmission of dharma idea comes from a story wherein the Buddha called together all of his cronies. Everyone was prepared for a heavy sermon, but all the Buddha did was hold up a flower. Only Mahakashyapa "got it", and so the story goes that this was the beginning of Zen.