Lord Vishnu is prominent as the Preserver of the universe. Vishnu's restoring, shielding and conserving powers have been apparent in the world in a sequence of ten mortal incarnations known as Avatars. The avatars appear in order to balance the effect of good and evil upon the earth. It is believed that nine avatars have already descended, three in non human form, one in hybrid form and five in human form. Of the ten universally recognized avatars, nine have already appeared whereas the tenth is yet to appear. These ten avatars are also known by the name of Dasha Avatar of Vishnu.
All the Avatars are mortal form of Lord Vishnu, who himself is eternal, consistent and undeniable. The Dasha Avatar of Vishnu are briefly discussed here:
Matsya Avatar - Lord Vishnu in the form of Fish
The first avatar, Matsya, was taken by Lord Vishnu at the end of the Satyuga (last age), when a flood destroyed the world. Through this avatar, he saved humanity and the sacred Veda text from the flood.
Kurma Avatar - Lord Vishnu in the form of Turtle
The second avatar, Kurma, was taken in the Satyuga (last age) to help the Devas and to obtain the amrut (nectar) of immortality which was also sought after by the Asuras (demons). He helped in creating the world by giving support of his back through this avatar.
Varaha Avatar - Lord Vishnu in the form of Boar
The third avatar, Varaha, was taken at the end of the last flood in the Satyuga, when Bhoomi Devi (Earth Mother) sank to the bottom of the ocean. Vishnu, in the form of varaha, dived into the ocean and raised the goddess out of the ocean, supported by his two tusks.
Narsimha Avatar - Lord Vishnu in the form of Half - Man and Half - Lion
The fourth Avatar, Narsimha, was taken in the Satyuga (last age) to kill a tyrant demon king. Narsimha is the only avatar which was Hybrid in form being half human and half animal.
Vamana Avatar - Lord Vishnu in the form of a Dwarf
The fifth avatar, Vamana, appeared in the Tretayuga in order to destroy Bali, the king of demons. He came during a huge ceremony conducted by the king and cleverly asked for just three feet of land, measured by his own small feet. Vamana covered whole of the earth and the heaven, subduing Bali into his feet.
Parshurama Avatar - Lord Vishnu in the form of a Brahmin
The sixth avatar, Parshurama, appeared in the Tretayuga to destroy the warrior caste. When the kings of the earth became despotic and started to harm people and saints in the forest, Vishnu incarnated as Parshurama and destroyed all the kings who were harassing the people.
Rama Avatar - Lord Vishnu in the form of the King
The seventh avatar, Rama, the prince and king of Ayodhaya, appeared in the Tretayuga, to rescue Sita with his loyal servant Hanuman and his brother, Lakshmana, and killed the demon Ravana. Lord Rama became one of the most popular gods in the Hindu religion.
Krishna Avatar - Lord Vishnu in the form of a Cowherd's Boy
The eighth avatar, Krishna, along with his brother Balarama, appeared in the Dwaparyuga to kill the demon king, Kansa. Lord Krishna conveyed the message of love and humanity to the world. Krishna told the epic poem Bhagavad Gita to the warrior Arjuna ,in which he acclaimed : ''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.''
Buddha Avatar - Lord Vishnu in the Form of Buddha
The ninth avatar, Mahavira Buddha, appeared in the Kalyuga, to teach the lesson of following a middle path in life. ''Buddha'' means 'the enlightened one'. He taught that all sorrow comes from attachments and desires, so it's better to curb all attachments in order to remain happy.