Dasavatharam Describes the Story of Ten Incarnations of Lord Vishnu.There are Ten Avathars of Vishnu in Dasavatharam.All these Ten Avathars are Formed for Protecting Dharma(Morality) on Earth.There are Two Concepts of Dasavatharam.
1)North Indian Concept
&
2)South Indian Concept
1) North Indian Concept Based on Dasavatharam:
In North India The Popularity of Lord Buddha is Very High.So North Indian Belives that ,In the Time of Nineth Incarnation Lord Vishnu Incarnated as Lord Buddha,In order to Save Hinduism from Excessive Idol Worships & Rituals.
Story of the Incarnation of Lord Buddha
In the Kaliyuga the demons were completely subjugated by the deities. Shukracharya the teacher of the demons instigated the demons to perform Yagya so that they could regain power and authority. Fearing this the deities prayed to Lord Vishnu for help. Lord Vishnu took incarnation as Buddha and dissuaded the demons from performing Yagya as it involves violence the demons stopped performing Yagyas.
2) South Indian Concept Based on Dasavatharm:
Incrnation of Lord Balaram
In South India Lord Balaram (The Elder Brother of Lord Krishna ) is Regarded as an Avathar of Lord Vishnu.In The Ten Avathars of Lord Vishnu -Lord Balaram is Concidered as the Eighth Avathar.In this incarnation Lord Vishnu killed Pralambasura the demon and he killed many more demons.(In North India Lord Balaram is Regarded as the Avathar of Lord Vishnu's Snake Anantha Shesha)