The Gayatri Mantra consists of twenty-four syllables - three lines of eight syllables each. The first line (Aum Bhur Bhuvah Swah) is considered an invocation, and is not technically a part of the original Gayatri Mantra as it appears in the Upanishads. Gayatri is also referred to as a Vedic poetic meter of 24 syllables or any hymn composed in this meter. Hence, there exists a whole family of Gayatri Mantras, which serve as meditative aids to pray for the blessings of a particular personal God.
Aum Bhur Bhuvah Swah, Tat Savitur Varenyam
Bhargo Devasya Dhimahi, Dhiyo Yo Nah Prachodayat
A basic translation can be given as...
Oh God, the Protector, the basis of all life, Who is self-existent, Who is free from all pains and Whose contact frees the soul from all troubles, Who pervades the Universe and sustains all, the Creator and Energizer of the whole Universe, the Giver of happiness, Who is worthy of acceptance, the most excellent, Who is Pure and the Purifier of all, let us embrace that very God, so that He may direct our mental faculties in the right direction.