The Avatar concept is a corner stone in Hindu philosophy. Avatar is a descent or a direct incarnation of God to ascent the mankind. Periodically the Supreme Lord Vishnu descends upon the earth, in animal or human forms and the mission is to preserve the world from the increasing evil. There are 10 avatars of Vishnu.

"Yada Yada Hi Dharmasya
Glanirva Bhavathi Bharatha,
Abhyuthanam Adharmaysya
Tadatmanam Srijami Aham'.
Bhagavad Gita
(Chapter IV-7)

Narrates Krishna to Arjuna : Whenever Dharma, or the situation of law and order, is endangered on this world, I incarnate onto this world to re establish Dharma, law and order, and to protect the Sadhus or saints and to destroy the evil elements of the society. Bhagavat Gita

In the earliest yuga (era) of Sata-yuga, a king named Manu was performing severe penance for thousands of years. One day as he was performing ablutions with river water, a small fish came into his hands and just as he was about to throw the fish back into the river, the fish requested the king to save its life. Heeding its request, the king put the fish into a jar of water but the fish started growing and the jar was not big enough for it. Then the king threw it into the river, but it soon it outgrew the river and the king then threw it into Ganges and then into the ocean. The king realised that it was Lord Vishnu himself and then the lord made an appearance and made a special request to the king. It predicted that the world would come to an end by a huge flood in seven days and requested the king to build a huge boat and take the seven sages(hermits), seeds of all plants, one animal of each type and told him that he would appear as a fish to propel the boat to Mount Himavan for surviving the flood to the next yuga(eon).True to his word, after seven days the Lord appeared and the king tied the boat to the fish by using the royal serpent Vasuki and the fish took all of them to Mt Himavan and kept them there till the flood was over and in the new era, the king started procreation a for the new era