Rahu is actually not a planet. In reality, it is an intelligent concept which is unique to Indian astrology. The two nodes where the trajectory of Earth round the Sun, and the trajectory of Moon around the earth intersect are named as Rahu and Ketu.
"Ardha-kayam maha-viryam chandraditya-vimardanam
simhika-garbha-sambhutam tam rahum pranamamy aham"

The above lines are in praise of the Rahu Planet and it tells that the worshipper by bowing down to Rahu praises the famous warrior who splits even the Sun and Moon in two half, born from Simhikaa` womb.

Origin of Rahu in Indian Astrology
In the old scriptures on Vedic astrology, there is no mention of Rahu. Rahu`s name appeared first in the great epic of Mahabharata, when the churning of the Ocean of Milk is described. Churning of Ocean is done to find ambrosia. Ambrosia is the elixir of immortality and it is called `amrita` in Sanskrit Language. Rahu was Danava Vipracitti`s son and his mother was Simhika. Maya, a Danava or Rakshasa, the famous architect and magician was his brother.

Myths about Rahu
According to mythology, when Lord Vishnu extracted the nectar of immortality by churning the sea, he invited Gods and Gods only, to drink it. Rahu disguised himself as God and drank the nectar, sitting between the Sun and Moon Gods. Hence Vishnu cut his head. Since the nectar had reached the neck of Rahu by that time, the head survived, while the rest of the body turned to ashes. Since then, both the planets are hell bent on taking revenge with Moon and Sun, by constantly causing Lunar and Solar eclipses.

Effects of Rahu
Rahu is regarded as a Shudra graha, which rules illiterate and uncultured people. In horoscopes, it is considered to be a malefic planet, which signifies thieves, witches, venom, magicians, snake, jails, spies, criminals, cemeteries, terrorists, etc. People dealing in occult, mainly dark magic are deeply influenced by this planet. Rahu is similar to Saturn in its effects.

Rahu is instinctual and animalistic in nature. It craves for pleasures, and is never satisfied with the gains. Fame, success in politics, money and physical attractions are all granted by this planet.