प्रस्तावना

भगवान श्रीकृष्ण अर्जुनाला म्हणाले,
यदा यदा हि धर्मस्य ग्लानिर्भवति भारत
अभ्युत्थानमधर्मस्य तदात्मान.म सृजाम्यहम
परित्राणाय साधुनाम विनाषाय च दुष्कृताम
धर्म संस्थापनार्थाय संभवामि युगे युगे
भगवान श्रीकृष्ण अर्जुनाला म्हणाले, हे भारता(भरतवंशी अर्जुना), जेव्हा जेव्हा धर्माचा ऱ्हास आणि अधर्माची वाढ होत असते, तेव्हा तेव्हा मी आपले रूप रचतो म्हणजेच आकार घेऊन लोकांसमोर प्रकट होतो.सज्जनांच्या उद्धारासाठी,पापकर्म करणाऱ्यांचा नाश करण्यासाठी आणि धर्माची उत्तम प्रकारे स्थापना करण्यासाठी मी युगायुगात प्रगट होतो.
श्रीविष्णुने सज्जनांच्या उद्धारासाठी या पृथ्वीतलावर दहावेळा अवतार घेतले.याचेच वर्णन महाभारतात भगवद्‌गीतेमधून श्रीकृष्णाने अर्जुनाला सांगितले आहे.

Bhagavan Shrikrishna explained Arjuna about 'Avatars'in Mahabharat through Bhagavadgeeta.
"Whenever Dharma, or the situation of law and order, is endangered on this world, I shall incarnate onto this world to re-establish Dharma, law and order, and to protect the good people and to destroy the evil elements of the society."

Vishnu is the name given to the protector and sustainer of the universe. Lord Vishnu, has incarnated in various life forms through different yugas (ages or eons) in situations where Dharma was in danger, because of certain evil elements in the world. There are ten incarnations of Lord Vishnu. These incarnations are termed as the 'avatars' of Lord Vishnu. Each avatar of Lord Vishnu shall be presented below with appropriate details of the situation under which the lord was compelled to appear on the earth.

While many explanations are given for the 10 avatars of Vishnu, one can see striking similarity of these stories with the theory of evolution of life.
In MATSYA Avatar, Lord Vishnu incarnates himself as a fish in this world
In KURMA Avatar, Lord Vishnu incarnates himself as a turtle.
In VARAHA Avatar, Lord Vishnu incarnates himself as a boar in this world.
In NARASIMHA Avatar, Lord Vishnu incarnates himself as a semi-man, semi-lion in this world.
In VAMANA Avatar, Lord Vishnu incarnates himself as a dwarf priest in this world.
In PARASHURAMA Avatar, Lord Vishnu incarnates himself as a brahmana (priest) in this world.
In RAMA Avatar, Lord Vishnu incarnates himself as RAMA, the central character in the epic Ramayan.
In KRISHNA Avatar, Lord Vishnu incarnates himself as KRISHNA , the central character in the epic MAHABHARATA
In BUDDHA Avatar, Lord Vishnu incarnates himself as BUDDHA, the ascetic prince who renounced the throne to lead the world on the path of peace
In KALKI Avatar, Lord Vishnu will incarnate himself as KALKI, the machine-man, who will come riding his white horse and with his blazing sword in his hands.