AUM consists of three phonemes A, U, and M. AUM represents the Hindu Trimurtior Trinity-A: Brahma representing the principle of creation, U: Vishnu, the principle of sustenance,M: Mahesh or Shiva-the principle of dissolution for further creation. AUM also represents the three Goddesses or forms of Shakti-energy associated with the Trimurti. AUM represents three Gunas, attributes: Sattvika-light, purity and serenity, Rajasika-activity, heat and fire, and Tamasik-dullness, darkness and ignorance. It also represents the three stages of existence: birth, life and death.
AUM symbolizes the manifestation of the formless Absolute Reality,Brahman, in form and sound. AUM symbolizes the oneness in the multiplicity of existence. AUM represents the triple appellation of the Brahman- that is beyond the beyond - beyond time, space and causation.
AUM is used as a mantra by itself. It is called Bija (seed) or Moola (root) mantra. AUM represents four states of consciousness: A-Jagruti (wakefulness), U-Swapna(dreaming),M-Sushupti(deep sleep) and Silence-Turiya- the Fourth (transcendental state of consciousness).