OM (AUM)

NOTE: In above pic, Jagrat & Sushupti are interchanged. Thanx Vallabh for pointing that out to me. On second thoughts, I should stick to inspired stuff, original is not meant for me ;)

The large lower curve symbolizes the waking state (jagrat). In this state the consciousness is turned outwards through the gates of the senses. The larger size signifies that this is the most common ('majority') state of the human consciousness.

The upper curve denotes the state of deep sleep (sushupti) or the unconscious state. This is a state where the sleeper desires nothing nor beholds any dream.

The middle curve (which lies between deep sleep and the waking state) signifies the dream state (swapna). In this state the consciousness of the individual is turned inwards, and the dreaming self beholds an enthralling view of the world behind the lids of the eyes.

These are the three states of an individual's consciousness, and since Indian mystic thought believes the entire manifested reality to spring from this consciousness, these three curves therefore represent the entire physical phenomenon.

The dot signifies the fourth state of consciousness, known in Sanskrit as turiya (shown as bindu in the above pic). In this state the consciousness looks neither outwards nor inwards, nor the two together. It signifies the coming to rest of all differentiated, relative existence. This utterly quiet, peaceful and blissful state is the ultimate aim of all spiritual activity. This Absolute (non-relative) state illuminates the other three states.

Finally, the semi circle symbolizes maya (shown as Raif in the above pic) and separates the dot from the other three curves. Thus it is the illusion of maya that prevents us from the realization of this highest state of bliss. The semi circle is open at the top, and does not touch the dot. This means that this highest state is not affected by maya. Maya only affects the manifested phenomenon. This effect is that of preventing the seeker from reaching his ultimate goal, the realization of the One, all - pervading, absolute principle.

This is the simplest analogy I understand and can explain as well. However, there are more analogies that have been explained. [Whew…those are too much to read at one go.]

For more reading:
http://om.exoticindiaart.com/
http://www.mandalayoga.net/index.php?rub=newsletter_en&p=mantra_om