What is Zen?

Zen Buddhism originates in the teachings of Siddhartha Gautama. Around 500 B.C. he was a prince in what is now India. At the age of 29, deeply troubled by the suffering he saw around him, he renounced his privileged life to seek understanding. After 6 years of struggling as an ascetic he finally achieved Enlightenment at age 35. After this he was known as the Buddha (meaning roughly "one who is awake"). In a nutshell, he realized that everything is subject to change and that suffering and discontentment are the result of attachment to circumstances and things which, by their nature, are impermanent. By ridding oneself of these attachments, including attachment to the false notion of self or "I", one can be free of suffering.
(learn more about Zen from altzen.com)

What does "ohm" (or aum) mean?

Aum is a mystical or sacred syllable in the Indian religions. It is placed at the beginning of most Hindu texts as a sacred exclamation to be uttered at the beginning and end of a reading of the Vedas or previously to any prayer or mantra. The Mandukya Upanishad is entirely devoted to the explanation of the syllable
(Learn more about Ohm from wikipedia) .