Aum (also Oṃ, written in Devanagari as ॐ and as औम्, in Sanskrit known as praṇava प्रणव [lit. “to sound out loudly”] or auṃkāra (also as Omkāra) ओंकार (lit. “auṃ syllable”) is a mystical or sacred syllable in the Indian religions, i.e. Hinduism, Buddhism, and Jainism.
Aum is commonly pronounced as a long or over-long nasalized close-mid back rounded vowel, [õːː]) though there are other enunciations pronounced in received traditions. It is placed at the beginning of most Hindu texts as a sacred incantation to be intoned at the beginning and end of a reading of the Vedas or prior to any prayer or mantra. The Māndukya Upanishad is entirely devoted to the explanation of the syllable. The syllable is taken to consist of three phonemes, a, u and m, variously symbolizing the Three Vedas or the Hindu Trimurti or the three stages of life ( birth, life and death ).
… Thanks wikipedia. :)