Vishnu is one of the great three Hindu
gods, the trimurti
. Vishnu sleeps on a great serpent, named Ananta
("Endless") in an ocean of milk
. These three represent the spirit that stirs the universe into action. Thus, Vishnu is called the divine dreamer whose dreams are that what happens.
Some believe Vishnu has and will appear as an avatar exactly ten times, of which nine times have passed; his avatars were the fish Matsya, the tortoise Kurma, the boar Varaha, the man-lion Narasinha, a dwarf, then Rama, then Krishna, and probably Buddha, and finally (closing the circle of life), Kalki. This is better covered in the Incarnations of Vishnu.
(Sources) I have looked on www.pantheon.org and have been pointed to http://psych.csuci.edu/psy344/JosephCampellOnHinduism.pdf
While Hindus have three neatly sorted gods, of which Vishnu is one, other religions have explained how evil can come into being by a split of the godhead, cf. Valis by Philip K. Dick.