"The Vedas and Puranas are one and the same in purpose. They ascertain the Absolute Truth, which is greater than everything else. The Absolute Truth is ultimately realized as the Absolute Personality of Godhead with absolute controlling power. As such, the Absolute Personality of Godhead must be completely full of opulence, strength, fame, beauty, knowledge and renunciation."
Srila Vyasadeva, in kindness to the fallen conditioned souls, supplemented the Vedas with the Puranas, which easily explain the Vedic truths in a way that all people can understand. The Puranas relate the Vedic teachings to historical facts and events that explain the teachings of the Rg, Sama, Atharva and Yajur Vedas. The Puranas, along with Chandogya Upanisad and the Mahabharata, are often referred to as the fifth Veda.
The Itihasas are puranic literatures describing historical events and pastimes of a single hero, or a group or lineage. For example, the Ramayana describes the pastimes of Sri Ramacandra, and the Mahabharata describes the pastimes of the Pandavas in the Kurus lineage. The Bhagavad-gita is part of the epic Mahabharata, and instructs that one surrender all other activities and engage fully in devotional service to Lord Krsna. Similarly, the Ramayana teaches surrender to the lotus feet of Lord Sri Ramacandra.
The 18 Primary Puranas
There are 18 primary Puranas, 18 upa-puranas, and numerous minor Puranas. The primary Puranas describe these five subjects: sarga (creation), pratisarga (recreation), vamsa (history of the sages), manvantara (periods of Manu), and vamsanucarita (geneology of kings).
*Lord Visnu is the predominating Deity of the following six Puranas:
Bhagavata Purana: has 18,000 verses, and is a jewel of the Vedas, providing all instruction on the science of self-realization.
Visnu Purana: has 23,000 verses, including stories of various devotees, a description of varnasrama, the six angas of the Veda, a description of the age of Kali, a description of Sveta Varaha Kalpa, and Visnu dharmotara.
Naradiya Purana: has 25,000 verses, and contains a synopsis of the complete philosophy. It describes Jagannatha Puri, Dwaraka, Badrinatha, and other holy sites.
Padma Purana: has 55,000 verses, and contains the glory of Srimad-Bhagavatam, and stories of Rama, Jagannatha, Matsya, Ekadasi, Bhrgu, and others.
Garuda Purana: has 19,000 verses, and discuss the subject of Bhagavad-gita, reincarnation, visnu-sahaasra-nama, and a description of Tarsya Kalpa.
Varaha Purana: has 24,000 verses, and describes the different vratas and Lord Visnu's glories.
*Lord Brahma is the predominating Deity of the following six Puranas:
Brahmanda Purana: has 12,000 verses, and describes the vedangas and the Adi Kalpa.
Brahmavaivarta Purana: has 18,000 verses, and describes the glories and pastimes of Sri Sri Radha-Krsna.
Markandeya Purana: ahs 9,000 verses, and tells the stories of Rama and Krsna.
Bhavisya Purana: has 14,500 verses, and describes the glories of devotional service to Sri Krsna, along with the prediction of Lord Caitanya.
Vamana Purana: contains 10,000 verses, and tells the story of Lord Trivikrama.
Brahma Purana: has 10,000 verses, and describes the teachings of Lord Brahma to Daksa.
*Lord Siva is the predominating Deity of the following six Puranas:
Matsya Purana: has 14,000 verses, and describes the Vamana and Varaha Kalpas, and temple construction.
Kurma Purana: has 17,000 verses, and describes the conversation between Krsna and the Sun-god, Dhanvantari, and the Laksmi Kalpa.
Linga Purana: has 10,000 verses, and describes the glories of Lord Nrsmhadeva, the stories of Janardhana and Ambarisa, and the glories of Gayatri.
Siva Purana has 24,000 verses, divided into six samhitas. It was recited by Vedavyasa’s disciple Romaharshana.
Skanda Purana: has 81,000 verses, and describes the slaughter of the demon Tarakdsura by Skanda (Subrahmanya). Skanda Purana is very similar to the Kumarasambhava of Kalidasa.
Agni Purana: has 15,400 verses, and describes Salagrama and the Isana Kalpa.
Source: Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.