Zen simply means "meditation." It is the practice of inquiry, the act of studying the self, of looking into the matter of our lives directly, of investigating the fundamental questions that arise from our sentience:

Who are we?
Why are we here?
Why do we suffer?
What is our correct role in this world?
How can we truly help?

In keeping these questions alive and open without containing them in tidy answers, we awaken to seeing things as they truly are, which is to say as constant change and flux. And in this openness arises our original nature, which is compassionate, concerned with alleviating the suffering of all beings.

Zen Master Dae Gak, in his book Going Beyond Buddha, describes entering this practice through the act of listening:

Listening is the fundamental practice of any spiritual path. By definition, to listen means to pay attention in order to hear, to heed, or to attend. In listening, we perceive things as they are. It takes no particular skill or understanding to listen. It only takes trying. Because we are humans, we are compassionate by nature. But our compassion becomes lost in self-interest. Listening is a practice that returns us to our true way. The way of human beings. The way of compassion.

Zen is also known as the direct mind-to-mind transmission outside of scripture and word. To study Zen is to take up this practice with a teacher who has been given this transmission to teach from a teacher who has been given transmission, and so on, tracing his or her lineage back to the historical Buddha Shakyamuni.