Vimshottari Dasha

Predictions in vedic astrology are based on dasha system and transits (Gochar) of planets, out of which dasha is of prime importance. 

Vimshottari dasha is dasha system based on the Nakshatra of the moon. Its 120 year long, which is sum of mahadashas of nine planets. Depending on the nakshatra you are born, you dasha will commence from its ruling planet.  Say your birth nakshatra is ruled by Jupiter, you dashas will start from Jupiter.

The main dasha is Mahadasha, which is sub divided into Antar dasha of each planet. So we have nine mahadasha; each mahadasha has nine antardasha; each antardasha is further sub divided in nine Pratya -antardasha, which further sub divides into sookshma-antardasha, and further into Prana-antardasha, and further into deha-antardasha.

The innermost dasha are extremely helpful in predicting future events with accuracy, but the birth time needs to be precise. For practical purposes, we stick to first three dashas - Maha, Antar and Pratya-antar dashas.

The mahadasha period for each planet in sequence - Ketu - 7 years; Venus - 20 years; Sun - 6 years; Moon - 10 years; Mars - 7 years; Rahu - 18 years; Jupiter - 16years; Saturn - 19 years; Mercury - 17 years. Same sequence is followed for antar dasha under each mahadasha. 

The outcome of the dasha of each planet depends on many factors. 

- Aspects like Badhaka or Mrityu Bhaga 

The dasha and sub dasha planet should not be in 6/8 position as its considered worst relative position for planets. Even 1/12 is undesirable but not as bad as 6/8 position. Any other positions of planets relative to each other is fine. 

One school of thought analyzes particular dasha's outcome taking mahadasha planet position as Lagna and then evaluating antardasha planet and its results counting all factors.