Zazen is a particular kind of meditation, unique to Zen, that functions centrally as the very heart of the practice. In fact, Zen Buddhists are generally known as the "meditation Buddhists." Basically, zazen is the study of the self.

The great Master Dogen said, "To study the Buddha Way is to study the self, to study the self is to forget the self, and to forget the self is to be enlightened by the ten thousand things." To be enlightened by the ten thousand things is to recognize the unity of the self and the ten thousand things. Upon his own enlightenment, Buddha was in seated meditation; Zen practice returns to the same seated meditation again and again. For two thousand five hundred years that meditation has continued, from generation to generation; it's the most important thing that has been passed on. It spread from India to China, to Japan, to other parts of Asia, and then finally to the West. It's a very simple practice. It's very easy to describe and very easy to follow. But like all other practices, it takes doing in order for it to happen.

We tend to see body, breath, and mind separately, but in zazen they come together as one reality. The first thing to pay attention to is the position of the body in zazen. The body has a way of communicating outwardly to the world and inwardly to oneself. How you position your body has a lot to do with what happens with your mind and your breath. Throughout the years of the evolution of Buddhism, the most effective positioning of the body for the practice of zazen has been the pyramid structure of the seated Buddha. Sitting on the floor is recommended because it is very stable. We use a zafu - a small pillow - to raise the behind just a little, so that the knees can touch the ground. With your bottom on the pillow and two knees touching the ground, you form a tripod base that gives three hundred and sixty-degree stability.

Here is an excellent video introduction to zazen meditation by Daido Loori Roshi.