Lord Maha Vishnu is the Supreme Godhead of Vaishnavism (Para Brahman or Nirguna Brahman) in puranas. He is called Swambhagwan in the Ramayana and Mahabharata. Vishnu is one of the three main Gods in Hinduism and is the Absolute Supreme being in the Vaishnava tradition. Vishnu is one of the members in the Trimurti in Hinduism.

Vishnu is the God of Preservation, genuinely meaning he protects the universe from being destroyed. It is believed that Lord Vishnu cradles the earth and all living organisms. According to the Hindu religion, he has set foot or arrived on the earth in nine forms called avatars, so far with one incarnation yet to come that is Kalki at the end of Kali Yuga to destroy evil. His most famous incarnations are Rama, Krishna, Parashurama and Narasimha.

Vishnu's wife is Lakshmi, the Goddess of Wealth and Fortune. Lord Vishnu is usually shown with a light blue skin and four arms. The Lord holds a lotus or Padma, the Indestructible Mace of Lord Vishnu (Kaumodaki), a conch (Shankha) and the Unstoppable Disc Of Lord Vishnu (Sudarshana Chakra) in each of his four hands. He is known to take many guises, but the three main forms of Vishnu are Mahavishnu Karanodakshayi Vishnu (Mahavishnu), Garbhodaksayi Vishnu and Kshirodakashayi Vishnu. The latter two are the expansion of Karanodakshayi Vishnu.

Avatars 
There are 10 Avatars of Vishnu (in the order they appeared):
 Matsya (Fish)
 Kurma (Turtle)
 Varaha (Boar)
 Narasimha (Half lion, Half man)
 Vamana (Dwarf / Short man)
 Parashurama (Fierce man / Hunter)
 Rama (Developed man / Ideal man / Fighter)
 Krishna (Cowherd Prince)

The diverse branches of the Hindu Tradition accept the ninety form of Vishnu as the following:

 Jagannath (Abstract Form)
Budhha (Enlightened Man)
 Kalki (future avatar, has not appeared yet)

Icons 
Lord Vishnu holds items in his four hands. The Lord holds a conch in the upper left hand, which represents victory. He holds the Sudarshan Chakra in the upper right hand, which represents spiritual energy. In his lower left hand, a lotus (flower) represents peacefulness, and in his lower right hand, the Kaumodaki represents the Lord's powers.

Temples of Vishnu (Mahavishnu)

Kodlamane Shree Vishnumurthy Temple : of Serenity and Mystery, is dedicated to Lord Vishnu and is the ancient Tirth Kshetra.  There is no other place like this anywhere in the Brahamanda.

Powers and Abilities 
 Note that all these abilities are from Bhagavata Purana and Vishnu Purana.
Omniverse manipulation/Absolute Manipulation/Meta Manipulation/Preservation: As the preserver, Vishnu can control anything and everything. The power to protect everything from anything is the reason why he is also called the Supreme protector of his devotees.

Absolute destruction: In the Vishnu purana, Vishnu is also described as the creator, destroyer, and preserver. As the destroyer, he can even destroy infinity or the omniverse.

Omni-Creation Or Absolute Creation: As the creator in Vishnu purana, Vishnu has the power to create everything, even infinity and beyond. The power to have limitless power upon seeing the Supreme form of lord Vishnu arjuna describes him having infinite prowess or power.

The Lord has the power to do absolutely everything, even creating something that is bigger than infinity itself. In Chapter 11 Of the Bhagavad Gita, Lord Krishna/Vishnu shows Arjuna everything—all of creation: time, space, past, present, future, omniverse, fictional and real human beings, and more. It is beyond human imagination to actually comprehend it.

Omniscience: The power to have infinite wisdom experience and knowledge.

Omnipresent: The power to be everywhere at every time including beyond the omniverse in both past present and future. When Lord Krishna/Vishnu shows his Supreme appearance, it is described being everywhere, yet people can't see it because they don't have the divine vision. Only Maharishis/sages, gods, Sanjaya, and arjuna are able to see this Vishvarupa.

Formless: Vishnu's true form is described as formless, infinite, boundless, inapplicable, without shape and color, according to Vishnu Purana.

Absolute immortality: Chapter 1 of Vishnu purana describes Vishnu being boundlessly above the concept of death idea and life.

Absolute Transcendence/Being Above All Things: The Vishnu Purana also describes Vishnu being above all descriptions such as logic, time, space, etc.                                       He is an infinite dimensional being according to Vishnu Sahasranamam.

References

Hindu gods and goddesses