The puranas were written to popularise the religion of the Vedas. They contain the essence of the Vedas.The aim of the Puranas is to impress on the mind of the masses rhe teachings of the vedas and 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.
Vyasa is the compiler of the Puranas from age to age, and for this age, he is Krishna- Dvaipayana, the son of Parasara.
Sarga or Creation, Pratisarga or Dissolution, Vamsa or the Geneologies of Kings or Sages, Manvantara - a duration of a reign of a Manu,Vamshanucaritam-the description of various Sages, Kings and incarnations. All the Puranas are strongly sectarian, some devoted to Shiva, some to Vishnu and some to a Goddess.
Lord Vishnu is the predominating Deity of the follwing six Puranas:
Vishnu Purana: It 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 Vishnudharmottara.
Naradiya Purana: It has 25,000 verses, and contains a synopsis of the complete Philosophy. It describes Jagannath Puri, Dwaraka, Badrinath and other holy places.
Padma Purana: It has 55,000 verses,and contains the glory of Srimad-Bhagavatam and stories of Rama, Jagannatha, Matsya, Ekadasi, Bhrgu and others.
Garuda Purana: It has 19,000 verses, and discuss the subject of Bhagavad-Gita, re-incarnation, Vishnu-Sahasranama and a description of Tarsya kalpa.
Varaha Purana: It has 24,000 verses,and describes the different vratas and Lord Vishnu's glories.
Among these Puranas the Bhagavata-Purana and the Vishnu-Purana are the most popular. So, these two have been dealt in detail here. The Bhagavata-Purana
The Bhagavata-Purana is considered to be the purest and the greatest of all the Puranas since it invokes devotion towards Lord Vishnu and his various incarnations, primarily focussing on Krishna since he was the complete incarnation of Lord Vishnu.
Vyasa delivered this story, which constitutes the cream he managed to extract from all the Vedic literature and histories, to his son Shukadeva, who is the most respectable one among the self-realized. Shukadeva told it to emperor Parikshit.It is a narration of a conversation between King Parikshit and Sage Shukadeva. (King Parikshit was cursed to die in seven days by Shamika Rishi's son Shringi. Maharaja Parikshit took it as a good opportunity to become indifferent towards all material things. With this in mind, he first of all handed over his kingdom to his son.Surrounded by the wise, he sat down at the banks of Ganges to fast until his death.)
This book tells the story of the Lord and his incarnations, since the earliest records of the vedic history.It is verily the Krishna Bible of the Hindu universe.It has 18,000 verses and consists of 12cantos/skandhas. These books tell the complete history of the vedic culture with the essence of all its classical stories called Puranas and includes the cream of the vedic knowledge compiled from all the literatures as well as the story of the life of the Lord Krishna in full(canto 10). It tells about His birth, His youth, all His wonderful proofs of His divine nature and the superhuman feats of defeating all kinds of demons up to great Mahabharata war at Kurukshetra.
Ten Avataras and their purpose
The Srimad-Bhagavatam is a chronicle of the various avataras of Lord Vishnu. The aim of every avatara is to save the world from some great danger,to destroy the wicked and protect the virtuous. The ten avataras are:
The object of the Matsya avatara is to save Vaivasvata Manu from destruction by a deluge. The object of Kurma avatara was to enable the world to recover some precious things which were lost in the deluge. The Kurma gave its back for keeping the churning rod when the Gods and Asuras Churned the Ocean of milk. The purpose of Varaha avatara was to rescue, from the waters, the earth which had been dragged down by a demon named Hiranyaksha. The purpose of Narasimha avatara was to free the world from the oppression of a demon named Hiranyakashipu(father of Prahlad). The object of Vamana avatara was to restore the power of the gods which had been eclipsed by the penance and devotion of King Bali. The object of Parashurama avatara was to relieve the country from the oppression of kshatriya rulers. Parashurama destroyed the kshatriya race 21 times. The object of Rama avatara was to destroy the wicked Ravana. the object of Srikrishna avatara was to destroy Kamsa and other demons and to deliver His wonderful message of the Gita in the mahabharata war. The object of Buddha avatara was to prohibit animal sacrifices and teach piety. The object of Kalki avatara is the destruction of the wicked and the re-establishment of virtue.
The basic message of Srimad-Bhagavatam is that every individual being is a part of the Lord but influenced by ignorance he is not able to recognize it. It is only by bhakti that one can get free of one's ignorance and attain to supreme love for Lord. Vishnu Purana
The Vishnu Purana is one of the 18 Mahapuranas. It has been given the name Puranaratna(Gem of the Puranas). It has been presented as a dialogue between Parashara and his disciple Maitreya and divided into six parts, the major topics discussed include creation of the Universe, stories of the avataras of Vishnu, stories of battles fought between asuras and devas, Geneology and stories of legendary Kings. A detail description of events in the life of Lord SriKrishna, starting from His birth, through His childhood, until the moment He left the earth and the destruction of the Yadava clan.The Vishnudharmottara purana, is a supplement or Appendix to the Vishnu Purana.
Vishnu Purana declares Vishnu/Narayana as Para Brahman or Supreme God who creates unlimited Universe and enters each one of them as Lord of Universe.God Narayana engages in creation of 14 worlds within the Universe as Brahma when he deliberately accepts rajas guna. God Narayana himself sustains, maintains and preserves the Universe as vishnu when he accepts sattva guna and annihilates the Universe at the end of Mahakalpa as shiva or Rudra when he accepts tamas guna. According to this reference, the holy Trimurti is non-different from God Narayana or Vishnu.