(The SH in this name is pronounced like 'sh' in 'she'. The N is pronounced like 'n' in 'turn'. )
One of the twelve Aadithyas is named Vishnu. Note to avoid confusion.
Lord Vishnu is the source of the creation of the universes. From him originated the creator Brahma and from the Brahma did originate the destroyer Rudra or Shiva. While Brahma creates, Vishnu maintains, Rudra destroys ( withdraws) the universe. These three are called Thrimurthis are Trinity of Lords. vishnu, Brahma and Rudra are said to be of the qualities sathva, rajas and thamas respectively.
Vishnu always defended the gods from the demons. He incarnates in different forms for different purposes at different ages of the time. Writing about Vishnu means writing the entire vedic literature.
His abode is called Vykuntham, a river before that is called Viraja, his consort is goddess of wealth Lakshmi,he lies in the milk ocean, his bed is the serpent Lord Anantha, his sons are Brahma and Manmatha, his bow is Saargna, his knife is Nandaka, his mace is Koumodaki,his conch is Paanchajanya, his disc is Sudarshana,his garland is Vyjayanthi, his medal is Kousthubham and his mole on the navel is Shreevathsam. He is variously known as Hari, Naaraayana, Vykuntha etc. He is mentioned and praised in the veda and puraanams too.
Basically Indian devotees fall into two broad categories, Vyshnavites and Shivites but the learned considered these two paths as one and the same. Teachers like Shankaraachaarya treated both these gods as equal and the same.