If Raamaayana and Mahaabhaarata mark the point of transition from Vedic religion based on sacrifices towards a personalized Godhead, Puranas gave the concrete foundation to Hinduism as it is mostly practiced today.
These texts in general deal with a variety of topics including the nature of creation Genesis, the series of creations and secondary creations, the Genealogy of the various rulers in various eras (yugas). But their emphasis lay in enumerating the greatness of the personal God, be it Shiva, Vishnu, Devi, or others elucidating the salient features concerned with the devotion and worship of the deities.
Traditionally, all the Puranas, the major one being eighteen in number are said to have been written by Veda Vyaasa himself. They are called the Mahaa Puraanas.
The Eighteen Mahaa Puraanas are :
1) Brahma Puraana 7) Maarkandeya Puraana 13) Skanda Puraana
2) Padma Puraana 8) Agni Puraana 14) Vaamana Puraana
3) Vishnu Puraana 9) Bhavishya Puraana 15) Koorma Puraana
4) Shiva Puraana 10) Brahma Vaivaarta Puraana 16) Matsya Puraana
5) Bhaagavata Puraana 11) Linga Puraana 17) Garuda Puraana
6) Naarada Puraana 12) Varaaha Puraana 18) Brahmanda Puraana
They give us a complete picture of the secrets of the universe and life.
The first six seek to present in a comprehensive manner the nuances of creation and their essential understanding, while the next four relate to us the real nature of the created world being ‘Vivarta’ i.e. “presentative/imagined” and not “factual/ real”. The following four Puraanas detail the nature of Lord Vishnu’s incarnation and show us the path to attain salvation. The following two deal with the nature of worldly beings, their origin, action, knowledge, spiritual practices, etc. The penultimate, Garuda in its final section gives a detailed picture of the “after-death scenario”. The final Brahmaanda Purana talks about the function of the individual in the world according to the results of the actions performed by him.
Thus we see that the unitary text “Purana” starts with Brahma and ends with Brahma and has Brahmavaivarta in the middle too, thereby emphasizing the fact that though religious ceremonies, etc. described in the text ought to be practiced, the underlying wisdom of the spiritual implications of the ceremonies will have to be kept in mind. Of the eighteen Puranas mentioned, the most popularly read Puraanas are Bhagavata (both Srimad Bhagavata as well Devi Bhagavata) Vishnu, Skanda, and Shiva.