Visa Regulation Valid in USA
The council of the United States requires everyone entering the country to issue a visa in the first place if needed. However, a Visa Waiver Program, valid on the territory of the United States since 1988, allows the citizens of some countries to come to the United States for no longer than 90 days without issuing a visa. Those countries are Australia, Andorra, Austria, Brunei, Belgium, Chile, Czech Republic, Denmark, Estonia, France, Finland, Germany, Greece, Hungary, Ireland, Iceland, Italy, Japan, Latvia, Luxembourg, Liechtenstein, Lithuania, Monaco, Malta, Netherlands, New Zealand, Norway, Portugal, San Marino, Slovakia, Slovenia, South Korea, Spain, Sweden, Singapore, Switzerland, Taiwan, United Kingdom.

To acquire that membership in that program, the one may go through the registration process online via the Electronic System for Authorization (ESTA). After the document is being issued, it will be valid for two years, allowing to come to the United States with specific purposes.
Notably, Canada, Cayman, Bermuda, Mexico, The Turks, and Caicos Islands citizens may enter the United States without a visa.
Some regulations need to be clarified for the citizens of those countries when entering the United States:
Bermudians and Canadians may enter the United States without a visa for no longer than six months;
The citizens who come to the United States under a visa-free program may stay in the country for no longer than 90 days;
Recently, it was decided that the citizens of the Marshall Islands, Palau, and the Federated States of Micronesia may come to the United States with the study or business purposes without issuing a visa if they provide a valid passport or any other TSA-approved identification document.
There are two types of a visa valid on the territory of the United States: Immigrant Visa that might be obtained for the permanent stay in the United States, and Non-Immigrant Visa that might be acquired with the purpose of the temporary visit in the United States.
The citizens of Libya, Iran, Syria, Somalia, Venezuela, North Korea, and Yemen were recently suspended from the right of the application for a visa under the president-signed proclamation of Donald Trump dated of December 4th, 2017.