The Vimshottari system

The former sages who founded the science of Jyotish, assigned an average length of 120 years to the human life. These 120 years are governed in turns by the nine planets, each one for its own period (mahadasha) that lasts a given number of years. A planet or another one, has a particular and more direct influence during some periods. For example, a person who has Venus well placed in the 11th house without malevolent aspect on it, can expect a good influx of income during its period. 
What do we mean by period? The ancient sages gave to every planet a number of years during which its influence becomes predominant on the individual. These major periods have been named "mahadashas" or big periods. Several systems of periods exist in the Vedic scriptures. The most commonly used is the "Vimshottari System" that generally gives the best results. This system is used to make some predictions, as to predict the moment where an event can happens, or to counsel a person to undertake or to avoid something when favorable or unfavorable periods are running. It also serves to predict periods more or less long, where the life of the individual will be dominated by particular influences. It also serves to rectify the birth time when it is imprecise. 
These big periods are divided into under-periods or what we call "antardashas". The antardashas also, can be divided in smaller periods and so forth, going from a level to another one, lower to it. Every sublevel, give details about the former one. It is like using in a microscope, more and more magnifications.