Lord Vishnu is also called as Dashavatara Moorthy, since he has taken various avatars to pacify the man kind, and the Dashavatara are the ten main avatars of Lord Vishnu, the protector god, and so far, Vishnu has taken nine out of ten avatars, and the remaining one would be taken only during the end of this Kali Yuga.
The list of the avatars of Lord Vishnu are: Matsya, Kurma, Varaha, Narasimha, Vamana, Parashurama, Rama, Krishna, Buddha and Kalki (Yet to take). Dashavatara replaces the Navagrahas in the temples of Lord Vishnu, since it is believed that by worshipping the Dashavataras of Lord Vishnu, all of our Navagraha Doshas would be removed, and we would get great satisfaction in our life.
As per Vishnu Purana, Lord Vishnu takes the various forms of Avatars, in order to remove the evil forces, and to maintain peace and prosperity in the world.
The first avatar Matsya was taken in order to save few of the pious people including Manu and Saptarishis during the time of deluge, since they were supposed to do their creation work.
The second Kurma avatar was taken during the time of churning of the divine ocean, Parkadal, in order to balance the Mandara Mountain, and also to avoid sinking it inside the Divine Ocean.
The third Varaha Avatar was taken, in order to rescue Ma Bhudevi from the Demon Hiranyaksha, and also to kill him.
The fourth Narasimha Avatar was taken to kill the demon Hiranyakasipu and also to save the child devotee Prahalada from the clutches of his father Hiranyakasipu.
The fifth Vamana Avatar was taken in order to give back the Swarka Kingdom to Lord Indra by pushing the great Mahabali inside the underworld, the Sutala Loka.
The sixth Parasurama Avatar was taken to remove the ego of the Kshatriya Kings, and to maintain Dharma in the world.
The seventh Rama Avatar was taken to destroy the demon Ravana, to spread the importance of “RAMA NAMA”, and about Hanuman Bhakti, and also to maintain peace in the entire world.
The eighth Krishna Avatar was taken to destroy certain evil kings like Kamsa and Shisupala, and also to give his divine teachings in the form of “THE HOLYBHAGAVAT GITA”.
The ninth Buddha Avatar was taken to implement strict non-violence in the entire world, and also to teach the principles of the holy religion Buddhism.
The tenth Kalki Avatar is not yet taken by Lord Vishnu, and it would be taken only during the end of this Kali Yuga.
“OM SRI DASHAVATARA MOORTHIYE NAMAHA”