Om

Bhuh Bhuvah Svahah
Tat Savitur Varenyam
Bhargo Devasya Dhimahi
Dhyo Yo nah Prachodayat
Om

The Gâyatrî Mantra is from the Rig Veda, which has existed in written form for at least 2500 to 3500 years. Some accounts date the original text to be over 6000 years old, passed down orally from generation to generation.

There are many translations of this revered chant into English. A favorite is this simple translation by S. Krishnamurthy:

We meditate upon the radiant Divine Light
of that adorable Sun of Spiritual Consciousness;
May it awaken our intuitional consciousness.

But to get inside of the gayatri it's helpful to reflect on the meaning of the words themselves, each of which holds layers of concepts.  Perhaps this is why the this mantra has been so revered for melinia: its meaning grows within us as our awareness and insight deepen.

A cursury look at each word in the Gayatri:

Om - Aum

From Amit Ray: "Om is not just a sound or vibration. It is not just a symbol. It is the entire cosmos, whatever we can see, touch, hear and feel. Moreover, it is all that is within our perception and all that is beyond our perception. It is the core of our very existence. If you think of Om only as a sound, a technique or a symbol of the Divine, you will miss it altogether. ..... Om is the mysterious cosmic energy that is the substratum of all the things and all the beings of the entire universe. It is an eternal song of the Divine. It is continuously resounding in silence on the background of everything that exists."

Bhur - eternal and unchanging existance

Bhuvah - absolute consciousness of Being

Swaha - all-pervading nature of Being

tat - that, referring to the Supreme Self (Buddha Self, Buddha Nature)

savitur - the all-pervading Consciousness

varenyam - most excellent, fit to be venerated

bhargo - glorious light of love and power

devasya - divine unity

dhîmahi - we meditate upon

dhiyo - understanding the nature of reality

yo - one who, a self-referential, reflexive, pointer to the one who meditates

nah - all of that which is, of which we belong

prachodayât - may the Unifying Consciousness energize, direct, inspire, and guide

There are many recordings of this chant available on the Internet.  One that may be especailly helpful for is this one (http://www.youtube.com/watch?v=dGFpd3mcVv0) which includes the words with the chant to make it easier to memorize.