According Hinduism whenever adhrama prevails over dharma on earth, Lord Vishnu incarnates himself on earth to destroy evil and protect good people. The famous Avatars of Lord Vishnu are the following ten known as Dasa avatar.
An asura named Hygriva stole Vedas from Lord Bhrama and he hide himself in the ocean. Bhrama approached Lord Vishnu for help. To get back the Vedas Lord Vishnu took the form of a fish. It was taken care by Sage Manu. When the fish grew bigger it went to the depth of the ocean fought with Hygriva and brought back the Vedas. At th is time There is a great flood on earth. Sage Manu and all species of living things are taken in boat and rescued by Matsayavatar of Lord Vishnu.
To get the nectar of immortality (amrit), devas and asuras were churning the ocean.They used the mountain Mandara as the churning staff and snake Vasuki as the rope. While they were churning, Mandara began to sink. At this juncture Lord Vishnu took the form of a giant tortoise(Kurma) and supported the mountain on his back and helped devas to restart the churning process and eventually get amrit .
Hiranyaska, an asura once stole the earth and hide it in the ocean. Lord Vishnu took the form of a wild boar and fought with the asura and brought back the earth and placed it in the universe.
Hiranyakasipu, an asura had secured a boon from Bhrama that he could not be killed by deva,asura,or human and his death would happen neither during day or night or inside or outside of a building. Having secured the boon Hiranyakasipu declared that as he was powerful than the gods everyone should worship him and not Gods. But his son Prahalada defied him and continued to worship Lord Vishnu, he tried his best to persuade Prahalada from worshipping Lord Vishnu. One day he became so angry that, he ordered the death of Prahalada. At this time to save Prahalada, Lord Vishnu came out from a pillar in the form of a half lion-half man and killed Hiranyakasipu by placing him on his thighs and piercing his body with claws..
King Bali, a descendent of Hirankasipu ruled earth. He was such a righteous ruler that Indra and the other Gods feared that may take over Heaven.They approached Lord Vishnu for help. Vishnu reincarnated himself as Vamana a Brahmin boy. One day while Bali was conducting a yajna, he approached Bali asking for alms. He asked for land under his feet in three steps. Bali agreed to grant his wish. Vamana then grew into a giant and placed his feet over earth and sky. Realising that Vamana is Lord Vishnu Bali allowed him to place his third step on his head. Bali was thus send to Pathal, nether world.
Parashurama was born as the son of rishi Jamadagni. Once King Karthvirayarjuna visited their ashram. Jamagani served the king with the help of divine cow Kamadenu. The King asked Jamadagni for the cow .Jamadagni refused but the King forcibly took away the cow and destroyed the ashram. An Angry Parashurama killed Karthavirayarjuna and brought back the cow. As a revenge The son of Karthviraarjuna killed Jamadagni.This made Parashurama vow that he will kill all Kastriyas and went on killing them. This stopped only after his grandfather made him stop killing.
With demons vandalizing earth, Bhoomi devi asked Lord Vishnu to help. He took his next human form as Sri Rama the king of Ayodhya. He killed the demon king Ravana and his army for abducting his wife Sita.
Krishna killed demon Khamsa and fought against evil at each step of his life.At the end he helps Pandavas win battle at Kurushetra. He also preached Bhagavat gita the holy text of Hindus to Arjuna. He established dharma over adharma.
Buddha is considered as an avatar of Lord Vishnu, he preached ahimsa and founded Buddhism.
The final avatar of Lord Vishnu, he is yet to appear and he will destroy all evil from the world.