Puranas

The Puranas and Itihasas are history of various time periods and Bhakti-centric. Elaborate directions are prescribed therein for performing Puja (worship), when Bhakti is translated into action. In addition to Puja, Yajnas (ritualistic sacrifices), Shraddhas (death anniversary rites) and Tarpan (homage to ancestors) are all considered indispensable adjuncts to Vedic Dharma.
Why Puranas –
The Vedic injunctions which are contained in the form of pithy statements are magnified or elaborated in the form of stories or anecdotes in the puranas. ‘Pura’ means in the past. Those that narrate things of the past are Purana. Puranas are indeed history. Eighteen Puranas add up to four thousand granthas or verses. A grantha is a sloka with thirty-two syllables. Of these, nearly one fourth or a hundred thousand slokas are taken by Skanda Purana. The balance of the seventeen Puranas contains three thousand granthas. In addition, Vyasa has produced the epic, “Mahabharata” which contains a hundred thousand granthas.
 
What are requirements of Puranas –
Purana has to fulfil five requirements in the matter – Sarga (original creation of the world), Prathisarga (how, after creation, the world grew with time), Vamsa (genealogy, how the descendants came from one generation to the next), Manvantara (the history of the fourteen Mnaus from whom all mankind descended, covering a period of one thousand 4-Yuga cycles) and Vamsaanucharita (the history of the rulers of the country, dynastic details like Surya Vamsa and Chandra Vamsa). In addition, it should also contain a description of this world in space. Here Purana acts not only as history but as geography too.
Each Purana keeps a single deity as its main theme. The Puranas are broadly classifiable into three categories – those which are prone to the Vaishnava cult, Shiva cult and the Shakti cult, thus covering a large number and variety of deities.
Why so many deities -
God has made men with different mental capacities and attitudes. God assumes various forms congenial to the respective mental predilections, in order that each may worship a desired deity and come to a good end. That is why the supreme Parmatama takes many forms as different deities.
Each one should have an unshakable faith in the chosen deity. They should be convinced that his deity is the supreme and ultimate Godhead and nothing can be above it. That is why in each manifestation, God shows himself as superior to the rest. The others are shown as worshipping the particular on and getting defeated in a conflict.
The Shaiva Puranas are merely a collection of those stories where Shiva’s supremacy alone is shown. The Vaishnava Puranas would be a compilation of incidents which glorify Vishnu to the subordination of others. So also, in the other Puranas. Thus, the intention is not to run down any particular deity, but to converge the attention of the devotee on that deity to the exclusion of others. The aim is to glorify a particular deity and heighten the devotion to that deity and not to vilify the others. This is for “Ananya Bhakti”.
 
How many Puranas –
The eighteen Puranas are - Brahma Purana, Padma Purana, Vishnu, Shiva, Shrimad Bhagavata, Narada, Markandaya, Agni, Bhavishyat, Bhrama Vaivartha, Linga, Varaaha, Skanda, Vamana, Koorma, Matsya, Garuda and Brahmaṇḍa Purana.

1. The Brahma Purana — The Purana recited by Brahma to Marichi is called the Brahma Purana, and it consists of 13,000 verses.
2. The Padma Purana - The Purana describes the cosmic Golden Lotus (from which the universe came out). It consists of 55,000 verses.
3. The Vishnu Purana— The Purana is recited by Parasara describes all Dharmas, beginning with a description of Varaha Kalpa. It consists of 23,000 verses.
4. The Shiva Purana — The Shiva or Vayu Purana describes fully the Dharmas, in the course of a description of the Sveta Kalpa, containing the glory of Rudra. It consists of 24,000 verses.
5. The Bhagavata Purana — The Bhagavata Purana is that which begins with Gayatri, and contains description of manifold Dharmas, together with the story of the death of Vritra. It gives an account of that Sarasvata kalpa. It contains 18,000 verses.
6. The Narada Purana — The Narada Purana is that wherein Narada tells all Dharmas of Brihat Kalpa. It contains 53,000 verses.
7. The Markandeya Purana — The Markandeya Purana is said to be the Purana told to the birds. It contains description of Dharma and Adharma and has 9,000 verses.
8. The Agni Purana — The Agni Purana is recited by Agni to Vasistha, describing the story of Isana Kalpa. It contains 16,000 verses and is the giver of the fruits of sacrifices.
9. The Bhavisya Purana — The Bhavisya is the Purana in which the four-faced Brahma, the Protector of the world, describes the greatness of Aditya, and in the course of the narration of Aghora Kalpa, recites to Manu, the creation and sustenance of the world and characteristics of various kinds of beings. It consists of 14,500 verses. It contains five Parvas: — 1. Brahma Parva, 2. Vaishnava Parva 3. Vajra Parva, 4. Tvastra Parva, 5. Pratibhasya Parva.
10. The Brahma Vaivarta Purana — The Brahma Vaivarta is that Purana in which beginning with the description of Rathantara Kalpa, Savarni tells Narada the glory of Krishna, and in which Brahma recites the actions of Varaha. It contains 18,000 verses.
11. The Linga Purana -That Purana which the God Maheshwara revealed standing in the column (Linga) of fire, that a man may attain Dharma, Kama, Artha and Moksha and which begins with a description of Agneya. Kalpa is the Linga Purana. It is so described by Brahma himself. It contains 11,000 verses.
12. The Varaha Purana — The Purana told by Visnu to the Earth, in the course of the description of Manava Kalpa, beginning with the praise of the Great Boar, containing 24,000 verses.
13. The Skanda Purana - The Purana told by the six-faced (Kartikeya) in the course of the description of Kim Purusha Kalpa, containing the Mahesvara Dharmas. It contains 81,101 verses.
14. The Vamana Purana — The Purana narrates about the glory of the Dwarf-Incarnation, Vamana during Kurma Kalpa. It contains 10,000 verses.
15. The Kurma Purana — Where in the course of the story of Indradumna, the Lord Janardana, in the form of a Tortoise, recited the greatness of the rishis. It contains 17,000 verses, and is relating to Lakshmi Kalpa.
16. The Matsya Purana — In the beginning of the kalpa, Janardana, with a view to promulgate the revelations (Shrutis), assuming the form of a fish described to Manu the story of Nara-Simha etc. It contains the account of seven kalpas. It has 14,000 verses.
17. The Garuda Purana — That which describes the birth of Garuda from the Cosmic Egg in the Garuda Kalpa and which is recited by Krishna. It contains 18,001 verses.
18. The Brahmanda Purana — The Purana begins with a description of the glory of the Cosmic Egg and which contains 12,200 verses; and wherein are the descriptions of many future Kalpas.

 
How many Upapuranas –
In addition to the eighteen Puranas, there are also eighteen “Upa” Puranas or auxiliary Puranas. Vinayaka Purana and Kalki Purana are among the Upa Puranas. Although they are said to be mainly eighteen in number, in actual fact, many more exist.
 
 
As per list from the Kurma Purana, the 18 Upapuranas are — recited by (I) Sanat Kumara, then (2) the Narasimha, (3) Kapila, then (4) the Manava (or Vamana), then (5) the Usanas, (6) the Brahmaṇḍa, (7) the Varuna, (8) the Kaiika, (9) the Maheshwara, (10) the Samba, the Saura, (12) the Parasara (the Pravara), (13) the Bhagavata, (Kurma 1, 15—20).
 
As per Parasara’s list — 1, the Sanat Kumdra, 2. the Narasimha, 3. the Nada, 4. the Shiva-Dharma, 5. the Daurvasa, 6. the Naradiya, 7. the Kapila, 8. the Manava, 9. the Usanas, 10. the Brahmanda, 11. the Varuna, 12. the Kali Purana, 13. the Vasistha, 14. the Lainga, 15. the Samba, 16. the Saura, 17. Parasara, 18. the Maricha, called also the Bhargava.
 
How many Ittihases -
The Ittihasses - Ramayana & Mahabharata are written by Valmiki and Vyasa respectively who actually witnessed the various incidents narrated. The itihasses are deemed to be as exalted as the Vedas themselves. Mahabharata is called the fifth Veda. As regards Ramayana, it is said that when the Purusha who can be known only by the Vedas took birth as Dashratha’s son, the Vedas also appeared as ‘Valmiki’s child in the form of Ramayana.  
Reference - The Vedas – by Pujasri Chandrasekharendra Saraswathi