The word ‘kala' in Sanskrit means death, and ‘Sarpa' means snake. This Yoga is formed when the planets are aligned such that seven of them come between Rahu and Ketu. Rahu is said to be a snake, while Ketu, its tale. If even one planet is not between them, then the Yoga is not formed. This dosha is said to be severe when all planets are within the arc from Rahu to Ketu in their natural motion direction.
If this Yoga is in someone's birth-chart, it does not mean that that person will become poor or will die young; rather it means that s/he will, despite all amenities and comforts, will live in constant stress and fear.
There are supposed to be 12 types of Kalasarpa doshas, based on the placement of Rahu from the lagna to the 12th house. Pandits had named them after the name of 12 ferocious snakes from India mythology.These are: Anant, Kulik, Vasuki, Shankhapal, Padma, Mahapadma, Takshak, Karkataka, Shankhanaad, Patak, Vishakata and Sheshanag. It is not known that when this theory of Kal Sarp Yoga emerged, but rest assured that it is none of the Rishis from Parashara to Varahamihira.
There are several remedies prescribed by astrologers to balance the effect of this yoga, that can be prescribed after the thorough examination of the horoscope of the person.