Lord Vamana is the fifth incarnation of Lord Vishnu.
The legend is that the mighty demon king Mahabali defeated Devas in a battle and he took possession of the devaloka. Devas became homeless and powerless.
This pained Mata Adithi- the mother of Devas. She requested to Sage Kashyap- her husband, to find a solution. He advised Mata Adithi to observe a special fast to please Lord Vishnu.
After observing this fast with devotion Lord Vishnu appeared to Mata Adithi. On her request, he took birth as Vaman on Mata Adithi’s womb.
Meanwhile the demon king Mahabali was doing a special yagna; he declared that he will grant any wishes of a pious Brahmin, in order to complete the Yagna successfully.
Hearing this, little Vamana dressed as a Brahmin boy, reached the place. King Bali welcomed his little guest with warmth.
King Mahabali then asked Brahmin boy to ask for anything that he can grand. The boy simply asked for three foots land. The King, who is the ruler of the world, thought this is too small a demand and granted it.
Immediately, the small boy became gigantic in size. He measured the entire earth on his first foot and the Patalaloka with his second foot. The boy then asked Mahabali to show him the place to measure the third foot.
A surprised Mahabali quickly realized who the boy is; so he happily permitted to measure the third step on his head. Lord Vaman agreed and put his foot on Mahabali’s head .
By the weight of it King Mahabali was pressed beneath the earth; he was sent to Patala Loka. This way Mahabali disappeared and Devas got their Devaloka.
Temples to Lord Vamana are very rare. However, there is a famous temple to him in Kerala, at Trikkaara near Kochi.
Here is the link