Bodhisattva who lived in or around the time of the Buddha. Known for his great compassion. Because of that, he is often called the Bodhisattva of Compassion.
He is believed to always live in human form, even in several different incarnations at the same time. So great is his compassion! For example, the Dalai Lama is believed by many to be Avalokitesvara.
In China, she is known as Kuan Yin (pinyin Guan Yin), in Japan as Kannon.
Yes, I used both he and she. That was not a typo. I once asked a Zen Master, "Kuan Yin... He or she?" He answered, "Yes."
Pronunciation note: The 's' in Avalokitesvara has a little dot over it (which I cannot reproduce here). It is pronounced half way between the English 's' and 'sh'. The 'v', as in all Sanskrit words, is pronounced as the English 'w'. A-wa-lo-ki-te-shwaa-ra.