The word purana means old or ancient. Thus the puranas are old and ancient texts that have come down to us through the ages. They have stories about famous people and descriptions of religion and society of those times. Amongst the Mahapuranas, the Markandeya Purana was one of the earlier ones to be composed with a date closer to 300 AD. With nine thousand shlokas, the Markandeya Purana is the shortest, and has no separate sections. It is divided into one hundred and thirty-seven chapters. There is very intimate connection between the Markandeya Purana and the Mahabharata. There are many unanswered questions that come to one's mind when one reads the Mahabharata, and some of these are answered in the Markandeya Purana.
The definition of a Purana is, in fact, quite precise. To be considered a proper Puranas, a text to has to cover five subjects. These are known as the five characteristics of a Purana. Traditionally, a purana must firstly describe the primary creation of the universe; this is known as sarga. But once the universe is created, it is periodically destroyed and created again. A purana must secondly describe this process of periodic destruction and creation; this is known as pratisarga. A purana must thirdly list out the genealogies of gods and saints, this is known as vamsha. Fourthly, a Purana must catalog the various manvantaras, this is, the many different eras that the earth or the universe has passed through. And finally, a Purana must have a history of the royal dynasties, vamshanucharita.
Around this core skeleton of the five subjects, any Purana normally contains matters of religious concern, customs, ceremonies, sacrifices, festivals, duties of the various castes, different types of donations, details of constructions of temples and images and descriptions of places of pilgrimage.
Eighteen mahapuranas are divided into three groups and each group has six texts. The Hindu Trinity consists of Brahma, Vishnu and Shiva. Brahma is regarded as the creator, Vishnu the preserver and Shiva the destroyer. Since all three are important gods, all these are given due emphasis in any Purana. But the relative emphasis often varies from Purana to Purana.
Texts which talk a lot about the incarnations of Vishnu are regarded as Vishnu Puranas, and are called Sattvika Puranas. Texts which emphasize creation more are regarded as Brahma Puranas and are called Rajasika Puranas. Texts which give a lot of importance to norms and rituals are regarded as Shiva Puranas, and are called tamasika Puranas.