What is Navagraha?
The Hindu astronomy, whose origins are in the Vedas, is based upon the configuration of the Nine(Nava) Planets (grahas) and their collective influence on the world in general and each individual in particular. Of the Navagrahas, Seven Grahas are named after the Sun, Moon and the first five planets in the Solar system not including the earth. Rahu & Ketu are considered the North &South Lunar nodes respectively. The first seven Navagrahas rule one day on the week. Rahu and Ketu are considered demons who managed to gain a place in the pantheon through an act of trickery.

Story of the trickery of Rahu & Ketu:
When the Devas asked Lord Vishnu for immortality, Lord Vishnu told the Devas to churn the oceans for "Amritha" or Nectar. As the nectar was being served to the Devas first, a demon disguised as a God sat in between the Sun & the Moon to received the nectar. The demon was recognized and Lord Vishnu immediately severed his head, but before his head was severed the demon had already consumed enough nectar to make him imoortal. The head of the demon is referred to as Rahu and the bottom part of the demon is referred to as Ketu. Ever since, Rahu and Ketu became the strong enemies of the Sun and Moon. Even though Rahu & Ketu are not divine, they have strengths of the Gods because of consuming the nectar and hence we treat them with the utmost respect.

Importance of the Navagraha:
The Navagraha play a vital role in deciding the course of our lives. The nine "grahas" or planets in our horoscope control our karma, our desires and their outcomes. Each of these nine planets exerts an influence in our lives, during a period which is called "dasa" and it can be known from one's horoscope. Every individual moves from one "Dasa" to another "Dasa" during which time that person's life is influenced by the "Graha" controlling that "Dasa". During theis time it is highly recommeded to chant the particular graha's Gayathri Manthra.