In the Bhagavad-Gita (4.8), Lord Krishna gives the reasons He descends to this world: "To deliver the pious and to annihilate the miscreants, as well as to re-establish the principles of religion, I Myself appear, millennium after millennium."
“Avtaaraa: asankhyeyah” – Avataaras are countless. There is a saying that the Lord Himself has not kept a tab on the number of Avataaras He had taken! Down from the Vedas to Puraanas, as many as 100 + Avataaras have been identified but only Dasha (10) as the main Avataaras.
The ten principal incarnations of Lord Vishnu are collectively known as the Dashavataras.
The following are the 10 principle incarnations of LORD VISHNU in 4 YUGAS:
4 Avataaras in Krita Yuga (Matsya, Koorma,
Varaaha and Nrisimha)
Matsya--the fish incarnation. He appeared in order to rescue the Vedas from the waters of devastation.
Kurma--the tortoise. He acted as a support for the mountain used in the pastime of churning the ocean.
Varaha--the boar. He picked up the earth and killed the demon Hiranyaksha.
Narasimha--the half-man, half-lion incarnation. He appeared in order to save His young devotee Prahlada.
Vamana--the dwarf. He tricked the demon king Bali into relinquishing control of the universe.
Parashurama--the warrior incarnation. He wiped out twenty-one generations of impious kings.
Ramachandra--the perfect king, hero of the Ramayana. He rescued his wife, Sita, from the clutches of the demon Ravana.
God of love and the ideal
becomes intimate union and communion with the incarnate God, both in His
timeless essence and in His ceaseless beneficent activity in the world of
Kalki--the destroyer. He will incarnate to wipe out the degraded population at the end of this age, the age of Kali or quarrel.