Rahu & Ketu are the nodes of the Moon
In Vedic Astrology they represent the transcendental nature of the self which is to be found in the periods when these planets are effecting us.
Rahu represents the head of a dragon which can never be full, where the lesson remains to find self restraint and discovering what fulfills us within the context of our true purpose.
Ketu symbolizes the body of the dragon which is already full and it cuts its own head as a symbol of cutting away all desires and worldly matters. It functions the best when it withdraws the senses away from the world and finds inner liberation or enlightenment.
Both are not actual planets, but are considered to be important in the Vedic system of astrology, since they represent certain karmic lessons of the soul. The dragon represents the awakening of the Kundalini Shakti within, the spiritual energy of the self, which needs to go through certain life lessons in order to be utilized and embodied on our paths of many lives.