In Hinduism, Indra is the king of the demigods or devas and rules the abode of demigods. In Vedas and early Vedic age, Indra was one of the most powerful gods but was never considered or related to the concept of Brahman or Supreme Truth, which is the guiding principle of Sanatana Dharma (Hindu religion). In the early Vedic age, he was mostly prayed to for the protection of cows and other wealth as he is believed to have controlled elements like rain, thunder etc.
The parents of Indra are sky god Dyaus Pita and the earth goddess Prthivi.
Indra is the leader of the Devas or demigods and the god of war, the god of thunder and storms. His weapon is Vajra, the lighting bolt or thunder. His vehicle or Vahana is Airavat, the white elephant.
In Hindu Puranas, Indra is constantly defeated by various demons and always runs to Brahma, Vishnu and Shiva for help. He is constantly thrown out of his kingdom by demons and is reinstated by the Trimurtis (Brahma, Vishnu and Shiva).
The end of the worship of Indra came during the incarnation of Vishnu as Lord Krishna. It was Krishna who asked the people of Vrindavan to stop praying to Lord Indra. An angry Indra sent torrential rains flooding Vrindavan but the people and their cattle were rescued by Lord Krishna who lifted the Govardhan hill and gave them shelter.
Symbolically, Indra was the personification of elements in Nature over which human beings had no control. But later developments removed the fear on Nature. This led to Indra losing power.
Bhogi Pongal festival observed in South India in January is associated with Indra and is known as Indra Vizha. But people no longer associate the festival with Indra.
Today, Indra is very rarely worshipped in Hinduism. He is mostly remembered in divine acts performed by Shiva and Vishnu to reinstate Him as the king of the demigods or devas.
Indra is the lord of senses (indriya = sensory organ in Sanskrit, meaning that which belong to Indra). This makes him the supreme of all the 33 devas or the essential spirits of the universe.
But sensory perception is limited in reaching towards the subtle truths of the universe. This is illustrated through the story of Uma who by her grace reveals to Indra about the supreme spirit Brahman when Indra is unable to comprehend what that is.
Indra is still worthy of praise and salutations (as can be often heard in the mantras Om namo Indraya) but he is not considered a deity worthy of worship as Ishwara. The concept of devotional bhakti and theistic worship arose late in India, during the period of epics, particularly with the worship of Krishna. The ancient relgious practices of the Vedas included sacrifice and austerity, but not worship of Ishwara.