What is the difference between an ESTA and a visa?

An approved ESTA is not a visa, it serves for only Tourism, Business or Transit. It does not meet the legal or regulatory requirements to serve in lieu of a United States visa when a visa is required under United States law. If an individuals possess a valid visa, they should travel to the U.S. on that visa for the purpose for which it was issued. Individuals traveling on valid visas are not required to apply for a travel authorization.