The difference between ESTA and a U.S. visa is:
1. ESTA is only for Visa Waiver travelers and is only related to “travel authorization”
2. A U.S. visitor visa is necessary for any other visitor not entering under the Visa Waiver Program.
ESTA is an online application system where the Visa Waiver traveler can apply for a travel authorization him or herself. An approved travel authorization is a new requirement to the Visa Waiver program since January 12, 2009 for entry to the U.S. via air or sea. However, an approved authorization does not guaranty entry at the U.S. border.
A U.S. visitor visa is either a B-1 Business Visa or B-2 Tourist Visa. The visa process can only be carried out abroad, and the visitor must physically apply at a U.S. embassy or consulate. The visa process involves several additional steps and costs in comparison to the ESTA. For the time being passengers can expect to also complete Form I-94W (the green form). Eventually ESTA will replace Form I-94W.