Hinduism

Aum, also written "Om" and called pranava, is a very important Hindu symbol and is often used to represent Hindu faith tradition as a whole. Its prolonged intonation is associated with the primeval sound through which the universe was created. It is thought to contain all things. It consists of three syllables — a-u-m — which are sounded progressively from the throat to the lips. 

The three sounds are considered to symbolize many items, but perhaps most importantly, the three states of consciousness: waking, dreaming, and deep sleep. The entire symbol represents the fourth state, which is the awareness of one's own spiritual identity.  

(Source:  http://www.hinduism.iskon.com)