India, being a gorgeous nation, attracts tourists from all around the world. If you're a US citizen, there's no demand for a visa to go into a country within US territory. If, however, you have to live and work in the United States, or if your country isn't part of the VWP, you're going to need a visa.
You may visit our area of Frequently Asked Questions to find more information concerning the ESTA. Let's say you're a British citizen and you're not really planning to remain in the usa for over 90 days, you just need an ESTA.