You do need documentation to enter India but you don't always need a Visa to enter India.
If you're a USA Citizen and are only wanting to go to India for tourist purposes and you plan to stay for only 60 days or less then you can apply for an electronic Travel authorization at least four days prior to arriving in India.
But if you plan to stay in India longer than 60 days then you'll need to apply for a Visa to enter India.