Literally Vishnu means "All Pervading God". God gives birth, preserves us and then takes away our lives. In this role, Vishnu is one of the three forms of Paramaatmaa - Brahmaa, Vishnu and Mahesh. In the form of Brahmaa He creates the world; in the form of Vishnu, He sustains the creation; and in the form of Mahesh or Shiv, He destroys His creation.

Although originally Vishnu was formless, the Puraan gave Him a human form with four hands resting on Shesh Naag on the ocean of Milk (Ksheer Saagar). He has Paanchjanya conch in one hand, a discus called Sudarshan Chakra in another hand, Kaumodakee Gadaa (mace) in His third hand, and a lotus flower in His fourth hand. Historically Paanchjanya conch and Sudarshan Chakra were held by Krishn. Later when Vishnu's four-armed figure was conceived these two items traveled from Krishn to Vishnu. He is said to have Shree Vats mark on His chest, He has a jewel on His wrist called Syaamantak Mani and another on His breast called Kaustubh Mani. His ride is Garud. A Pauraanik story goes to have a lotus come out of His navel on which Brahmaa sits. He lives in Vaikunth along with His consort Lakshmee.

At several places Vishnu's several forms have been mentioned. --

(Naarad Puraan, p 145) Puraan, Nyaaya, Meemaansaa, Dharm Shaastra and Ved including its six parts - all are called Vishnu's form.
(Naarad Puraan, p  ) Braahman is also Vishnu's form.

Padm Puraan gives the description of His Dhaam and guards like this :-

Mahaadev Jee said to Paarvatee Jee - "Vishnu's Param Dhaam is called Vaikunth Dhaam also. There are many states there. And in there are innumerable gem-studded Vimaan and palaces. There is a Divine city in the middle of that, which is called Ayodhyaa which is surrounded by boundary wall and high gates. There are pictures made from gems and gold.

Chand etc guards and Kumud etc Dikpaal guard it constantly. On its eastern gate are Chand and Prachand; on its southern gate are Bhadra and Subhadra; on its western gate are Jaya and Vijaya; and on its northern gate are Dhaataa and Vidhaataa. Kumud, Kumudaaksh, Pundareek, Vaaman, Shankukarn, Sarvnidra, Sumukh, Supratishthit - these are the Dikpaal of that city.

In the center of the city, there is a palace of Vishnu. In the center of that palace is a throm=ne Vishnu and Lakshmee are sitting in the center of an 8-petal lotus flower. In the center of that flower is the pistil, named Saavitree, and there sit Vishnu with Lakshmee Jee. Vishnu's complexion is like blue lotus flower. His all body parts are most beautiful. Lakshmee is sitting on His left side. She is also very beautiful. On 8 petals are sitting 8 Shakti - Vimalaa, Utkarshinee, Gyaanaa, Kriyaa, Yogaa, Pranhee, Satyaa, and Eeshaanaa.

Vishnu's Lok
Now you listen to His state. Maharshi tells His four Vyooh (dwelling places) - Vaikunth Lok, Vishnu Lok, Shwet Dweep and Ksheer Saagar. Vaikunth Lok is surrounded by water. It is the cause and so it is auspicious. It's light is like millions of fires. It is all religious. It is like Param Dhaam. Vaikunth Nagar is studded with various kinds of gems and is guarded by Chand etc guards and Kumud etc Dikpaal. It's length is 1,000 Yojan (8,000 miles) and breadth is 55 Yojan (410 miles). Millions of high palaces are beautifying it. All women there are like Lakshmee and men are like Vishnu. They all are related to Vishnu and they all enjoy like Vishnu. Whoever enters this Lok, he never comes back to Prithvi. Bhagavaan's Bhakt are free from all kinds of bondage, diseases and sorrows. Even those beings who live in Brahm Lok, take birth in the world, do Karm, and suffer with sorrows and fears. Because when their Punya decay, Devtaa get angry with them, and throw them to do Karm; that is why pleasures of Swarg is with sorrows. Vishnu destroys all sorrows that is why one should take His name only.

Here Vishnu sits in a Divine Vimaan. There are pillars studded with gems and there is a golden seat which is supported by Aadi Shakti. Upon that is an 8-petal lotus flower, also studded with various gems, and Naaraayan sits on that lotus flower. On His right side is Lakshmee Jee and on His left is Bhoo Devee. Vimalaa ets Shakti serve them from the 8 petals of the flower. This was the description of His one Vyooh" After this he described His other three Vyooh also - Vishnu Lok, Shwet Dweep and Ksheer Saagar.
[Padm Puraan, 5/36]

Aparaadh Towards Vishnu
Everyday people commit crimes towards Vishnu, so one should daily ask for His forgiveness. there are 32 crimes are told by Padm Puraan (Paataal Khand, p 564).     See     Aparaadh Towards Vishnu

Time of Vishnu's Sleeping and Waking up
When Soorya enters Kark Raashi (Cancer sign - on July Sankraanti), Bhagavaan sleeps; and 
When Soorya enters Tulaa Raashi (Libra sign - on October Sankraanti), Bhagavaan wakes up.

Vishnu's Preaching and Teachings
Vishnu Himself has taught many things to several people :-

Vishnu's 10 Avataar
Vishnu has taken 10 principal incarnations. See about His incarnations -  Avataar