The Gayatri Mantra may be the oldest chant that has been continually recited in the history of the world ('Pandit' David Frawley), with an antiquity of at least 5,000 years. Roughly translated: "We meditate upon the radiant splendor of the Divine solar Creator, that he may direct our minds". It is recited by both Buddhists and those in the Vedic tradition. For all Vedic mantras, there are three primary factors, and, for the mantra to be fully effective, it is said that one should know and honor all three factors. First is the deity to which the hymn is being addressed. Second is the meter, regarded as feminine in nature, and reflecting the Shakti of the hymn. Third, is the rishi who first received the hymn, usually in a state of deep meditation. One should chant the mantra according to an inner communion with the Devata, a feeling of the meter, or energy power of the chant, and a regard for the rishi.
The Gayatri Mantra is to Savita, who represents the Purusha/Consciousness within the sun (or the sun's rays) as symbolic of the Divine light, especially its transformative power. This is why the mantra is chanted at the transformative times of the sun's movement: sunrise and sunset. In Vedic thought, the sun is a source of Divine grace.
Somewhat confusingly, there are about 250 Vedic hymns in the Gayatri meter. Harish Johari's 'Sounds of Tantra' cd reproduces 25 of them - Shiva Gayatri, Durga Gayatri, for ex. However, the Gayatri mantra is apparently never mistaken for any other. The Gayatri meter represents the power of the Goddess to move through all difficulties, and causes her energy to arise within us.
The Gayatri mantra was revealed to/by the seer King Vishvamitra. He was originally motivated by anger, envy, and revenge, sidetracked by lust, and seduced by ego-bound ambition. Even after visitation by Brahma, he was still violence prone. Nevertheless, his effort was pure enough to overcome these obstacles, and he became a rishi. After sitting in meditation, he received the Gayatri.
At the highest level, the Gayatri opens our minds and heart to the Supreme Light, and grants Self-Realization.