In the basic Hindu Trinity of Brahma, Vishnu and Shiva, the Hindu god Vishnu is the preserver and protector of creation. Vishnu is the embodiment of mercy and goodness, the self-existent, all-pervading power that preserves the universe and maintains the cosmic order Dharma.
Vishnu is often represented resting on the coiled serpent Shesha, with Vishnu's consort Lakshmi massaging his feet. Vishnu never sleeps and is the deity of Shanti, the peaceful mood. Vishnu does not however tolerate ego.
Most often, the Hindu god Vishnu is shown with four attributes or weapons. In one hand Vishnu holds the conch or Sankha. The second hand of Vishnu holds the disc or Vaijra. The third hand of Vishnu holds the club and in the fourth hand Vishnu holds the lotus or Padma. Vishnu also has a bow called Sarnga and a sword called Nandaka.
Most of the time, good and evil forces are evenly matched in the world. But at times, the balance is destroyed and evil demons get the upper hand. Often in response to a request by the other gods, Vishnu then incarnates in a human form to set the balance right again. 9 Vishnu incarnations are generally recognized as Vishnu avatars, even though some sources also see other important figures of the indian epics as incarnations of Vishnu.

Following is an overview of the 9 principal avatars of Vishnu

First incarnation of Vishnu : Matsya or the Fish incarnation : in this form Vishnu saved the Saint Vaivaswata, the hindu variety of the biblical Noah (or vice versa).
Second incarnation of Vishnu : Kurma or the Turtle incarnation : at the Churning of the Ocean, Vishnu as Koorma (or Kurma) offered his back as a pivot on which to rest the Mount Mandara, used as a churning stick by gods and demons.
Third incarnation of Vishnu : Varaha or the Boar incarnation of Vishnu : he killed the demon Hiranyaksha, recovered the stolen Veda's and released the Earth from the bottom of the ocean.
Fourth incarnation of Vishnu : Narasingha or the Lion incarnation : as a creature who was half-lion and half-man, Vishnu killed the demon Hiranyashasipu, brother of Niranyaksha, who had gained the boon of immunity from attacks by man, beast or god.
Fifth incarnation of Vishnu : Vamana or the Dwarf incarnation : he killed the demon Bali, who had gained dominion over the Earth and had chased the gods from the heavens.
Sixth incarnation of Vishnu : Parasurama : he killed the King Kartavirya, who had stolen the holy cow Kamadhenu, which could grant all desires.
Seventh incarnation of Vishnu : Ram : he killed the demon King Ravana, who had abducted Sita. More on Ram can be read on the Ramayana page.
Eigth incarnation of Vishnu : Krishna : he killed Kansa, son of a demon and the tyrannical King of Mathura. More on Krishna can be read in the Life of Krishna.
Ninth incarnation of Vishnu : Buddha : Vishnu incarnated to remove suffering from the world. More on Buddha can be read on the Buddha page.
Tenth Incarnation of Vishnu : Kalki : still to come at the end of the Kaliyuga or the present age of decline, when Vishnu will appear in person on Earth, seated on a white horse, Kalki, which is his tenth incarnation.