Always a tricky question to answer simply, it may be best to remember that Zen is not an "is" so much as it is a "does". The word zen is actually Japanese rendering of the Chinese word chan, which in turn is a Chinese rendering of the Sanskrit dhyana. However you say it, Zen most directly means "meditate"; and in the Soto lineage of Zen, emphasis is placed upon the practice of sitting meditation above all else. It is viewed as the direct expression of enlightenment-- which our founder Dogen Zenji noted is not something one must attain, rather something to be directly expressed through the practice of zazen. In Soto, we face the wall as our ancestor Bodhidharma faced the wall of a cave. Our practice is supported by our posture: Erect spine, soft gaze, hands folded in the Zen mudra, legs solid, and breath continuous in an effortless effort.
What is Zen? The answer is one best experienced directly through one's own practice.