Meditation is the core of Zen practice. Zen literally means meditation. It comes from sanskrit and pali in India to the Chinese (Chan) to the Japanese. (see the word history in the Free Dictionary)
Zazen is the formal practice of Zen and it means sitting meditation.
Of course the question remains, "what is meditation?" or "what is Zen?" That questioning is part of the practice of Zen.
The posture of zazen. There are many different ways to do Zen meditation. During meditation our body and mind should be still. Depending on the individual, you may like to do sitting cross legged on the floor, kneeling on the floor, sitting in a chair, or even laying down. It is important to find a posture that is comfortable for you but also encourages you to empty your mind. If you are too comfortable it is too easy to sleep or day dream. If you are too uncomfortable the pain becomes distracting. Here are some good websites about how to do zazen:
Zen Center of Los Angeles
Zen Mountain Monastery
Santa Cruz Zen Center--Postures for People with chronic pain