Purana means stories or histories of ancient times. There are several subject matters in the puranas and they are classified into five sections. Their five sections are history, cosmology, secondary creation, genealogy of kings, and of Manvantaras or Manus. The Puranas are part of the Itihasas.
The Puranas were written to popularize the religion of the Vedas. They contain the essence of the Vedas. The aim of the Puranas is to impress on the minds of the masses the teachings of the Vedas and to generate in them devotion to God, through concrete examples, myths, stories, legends, lives of saints, kings and great men, allegories, and chronicles of great historical events. The sages made use of these things to illustrate the eternal principles of religion. The Puranas were meant, not for the scholars, but for the ordinary people who could not understand high philosophy and who could not study the Vedas.
Each purana gives prominence to a particular deity and therefore places all others on a subservient position. There are stories in the puranas to further explain the application of religion in life and to emphasize the importance of injunctions.
The Srimad Bhagavata Purana describes at length the appearance and activities of the ten main incarnations of Krishna. Other incarnations are also mentioned where necessary. The avatars appear to re-establish religious principles, to destroy those promoting irreligious life, and to enjoy pastimes with Their devotees. The ten Avatars are: Matsya (The Fish), Kurma (The Tortoise), Varaha (The Boar), Narasimha (The Man-Lion), Vamana (The Dwarf), Parasurama (Rama with the axe, the destroyer of the Kshatriya race), Ramachandra (the hero of the Ramayana who destroyed Ravana), Sri Balarama (brother of Sri Krishna), Buddha (the prince-ascetic, founder of Buddhism), and Kalki (the hero riding on a white horse, who is to come at the end of the Kali-Yuga).
Matsya saved Vaivasvata Manu and the Vedas when the waters of destruction overflowed the universe. Kurma used His shell as a pivot when the demigods and demons agreed to churn the ocean of milk to get the nectar of immortality. Varaha saved the world from the ocean below the universe by holding it on His tusk, when the world was thrown there by the demon Hiranyaksha. Narasimha appeared to save His devotee Prahlad from the oppression of his father, the demon Hiranyakashipu. Vamana appeared to trick King Bali by asking him some charity. Bali conquered the universe and Vamana restored it to the possession of the demigods.
Parasurama appeared to rid the world of the irreligious kshatriya kings. Parasurama destroyed the kshatriya race twenty-one times. Rama destroyed Ravana, who kidnapped His chaste wife Sita. Sri Balarama appeared to help Sri Krishna and removed the burden of the world by killing Kamsa and many other demons and kings. He also helped to re-establish religious principles and assisted Krishna to deliver the message of the Bhagavad Gita. Buddha taught compassion, vegetarianism and meditation to attain the Supreme Truth. Kalki will appear at the end of this age to destroy the sinful populace and to start again the golden age called Satya yuga.
The Jains have their separate Puranas. The most popular among them is the Mahapurana of Acharya Jinasena. A major part of the Jain Puranas can be found in Kannada literature. The Buddhist Purana, called Swayambhu Purana, is a major source of the history of the Kathmandu valley.