Short answers, pulled from the story.
Vishnu is a major deity in Hinduism who rose from a minor solar deity in the Rigveda to become the Supreme Being for hundreds of millions of Hindus. Early Vedic texts describe him as a close friend and ally of Indra who supports heaven and earth, while later texts like the Brahmanas assert that Vishnu is the essence in every being and all worlds are placed within his own self.
The Dashavatara or ten primary avatars of Vishnu include Matsya the fish, Kurma the tortoise, Varaha the boar, Narasimha the man-lion, and Kalki the warrior who appears at the end of the current age. Among these ten, Rama and Krishna stand out as the most significant, with Rama representing the ideal king and Krishna being venerated as the ultimate source of all existence in the Mahabharata and the Bhagavad Gita.
The reclining form of Vishnu known as Narayana depicts him lying on the coils of the serpent Shesha floating upon the primeval ocean of milk called Kshira Sagara while dreaming the universe into reality. This form includes four arms holding the conch shell Panchajanya, the discus Sudarshana, the mace Kaumodaki, and the lotus flower Padma to symbolize the interconnectedness of existence and the power to restore dharma.
In the Tamil language Vishnu is known as Mayon or Perumal meaning the dark one or the great one and was worshipped in the plains and mountains of Tamilakam according to the Sangam literature dating from 200 BCE to 500 CE. The Alvars were poet-saints who lived between the 6th and 9th centuries CE and traveled from place to place singing praises of Vishnu to establish temple sites such as Srirangam and Tirupati.
In the Bhagavata Purana Vishnu is said to breathe out the universe and then breathe it back in dissolving all things into himself before recreating them in a continuous rhythm of exhalation and inhalation. The Trimurti is the triple deity of Brahma Vishnu and Shiva where Vishnu is the preserver and the force of Sattva or goodness that maintains the balance of the universe.
The worship of Vishnu spread to Southeast Asia and was integrated into the religious cultures of neighboring civilizations where he is venerated as Upulvan in Sri Lanka and known as Bichu-ten in Japan. In Thailand Cambodia and Vietnam statues of Vishnu dating from the 4th to the 9th centuries have been found and in Indonesia the god is known as Wisnu a central figure in the wayang puppetry tradition.