Hindus not only worship cows and bulls, but they also protect them. Severally, Hindus seem to worship many elements, trees, sun, moon, water, cow, snake, elephant, and more. In Hinduism, God is visualized through these forms. According to Hindu Puranas, a cow is the form of Lord Vishnu. In Bhagavata Purana, the earth once converted itself in the form of a cow and told Lord Vishnu to protect her. Therefore, Lord Vishnu is called “Go-pala” by Hindus.
Besides, the cow is associated with the trinity Lord Shiva. “Nandi”–the bull is very fond of Mahadev and also the gatekeeper of Kailash. He is another well-known deity and a trusted vehicle of Lord Shiva. If you visit any temple of Mahadev, there will always be a statue of a cow, and people worship it. The cow is also an embodiment of Laxmi, the Goddess of wealth. Lord Krishna is also another form of a cow.
In the Vedic era, cows and bulls were the only sources of agriculture. The cow gives milk, which is a source of income, now and then. So, the cow was acknowledged as the form of God.
In Hinduism, the cow is a sacred animal. Dharmic (religious) people are quite sensitive about it. Where the rest of the world enjoy eating beef, in Hinduism, this activity is considered as a sin.