The featured Hindu god of the week this week is Vamana, the incarnation of Vishnu as a dwarf. The "dasavatar" stories, the stories of Vishnu's "ten incarnations (avatars)," are central to Hinduism. Vishnu is the god who preserves the universe, so when a crisis threatens the world, Vishnu takes an earthly form in order to save the earth from destruction. The incarnation of Vishnu that is probably best known to western audiences is his incarnation as Krishna.
The basic outlines of the story are very clear, although the versions told in the sacred Puranas add many details. The incarnation of Vishnu as Vamana, a dwarf, took place when the world was under the sway of Bali, a demon king (Bali was the grandson of Prahlada, the devoted worshipper of Vishnu made famous when Vishnu incarnated himself as the Narasimha, or Man-Lion).
The gods were feeling threated by Bali's power. Was Bali a bad ruler? The word "demon" has many negative connotations in English which do not necessarily apply here. In some versions of the story, Bali was actually a good ruler presiding over a "golden age," yet his arrogance in any case offended and threatened the gods of heaven.
In order to liberate the world from Bali, Vishnu, in the guise of a dwarf, went to Bali's court and humbly begged for some land. Bali generously offered to give grant Vamana's request, so the little Vamana asked for the amount of land that he could cover in three strides. Bali scoffed at the idea, and agreed. Then Vamana manifested himself in cosmic form, growing gigantically large. In his first step he covered all the space between heaven and earth, and with his second step he could all the space between earth and the lower world.
There was no room for him to even take a third step! Bali realized that he was in the presence of God, so he bowed down his head, and let Vishnu step on his head to complete the last step. That is what you see in the image below: if you look closely, you can see Bali piously praying as his head is stepped on by Vishnu, thus pushing Bali down into the underworld, the Patala Loka.
As often in Hindu stories, there is a sense of conversion when the demon king is defeated by the god. Because Bali piously kept his promise, accepted the power of the god and acknowledged his own defeat, Vishnu honored Bali by giving him the title "Mahabali," which means "Great Bali" (the Sanskrit maha- is etymologically related to the Greek mega-). To this day, Mahabali is celebrated at a religious festival in the Kerala region of India. You can read about the Onam festival at wikipedia.
You can read more about the story of Vamana and Mahabali at wikipedia. The image shown here depicts Vishnu "Trivikrama," "Three-Step," the third step being when he places his foot on Bali's head: