Generally, if you want to visit (and not live in) the United States you must first obtain a visitor visa. Travelers from certain countries may be exempt from this requirement. For more information, please see the U.S Department of State website, at www.state.gov.
Visitor visas are nonimmigrant visas for persons who want to enter the United States temporarily for business (visa category B-1), tourism, pleasure or visiting (visa category B-2), or a combination of both purposes (B1/B2).
You may be eligible for a B-1 visa if you will be participating in business activities of a commercial or professional nature in the United States, including, but not limited to:
Consulting with business associates
Traveling for a scientific, educational, professional or business convention, or a conference on specific dates
Settling an estate
Negotiating a contract
Participating in short-term training
Transiting through the United States: certain persons may transit the United States with a B-1 visa
Deadheading: certain air crewmen may enter the United States as deadhead crew with a B-1 visa