I found in Wiki very important:

A Dasha (Sanskrit दशा da¶ā) is a planetary period in Jyotisha, the classical system of astrology in India. The concept is claimed to provide the basis of an easy and comprehensive system to judge the effects of the planets throughout a person's life, by showing how the planets distribute their effects through time and the different stages of life. How each dasha or planetary period operates also depends on the position of the planet concerned in the individual's horoscope ie. whether it is placed in a beneficial or difficult position. The planets or graha are the nine navagraha: the seven classical planets known to the ancients, plus the north and south lunar nodes.

There are at least 43 different kinds of dasha. However, the most important dasha is the Vimshottari Dasha, a 120 year long cycle of planetary positions based on the position of the Moon in one of the 27 divisions or lunar mansions called nakshatra. Under this system, each planet is assigned a different mahadasha or period ranging from six to twenty years. For example, the mahadasha of Shukra (Venus), lasts twenty years and is believed to bring wealth, good fortune and luxury to the person who has it well positioned in their horoscope. Examples of other dashas include the Yogini Dasha, the Kalchakra Dasha, the Ashtotri Dasha and the Jaimini Dasha.