Visas are the means by which a foreign national gains permission to enter the United States.  The majority of visas are non-immigrant.  Such visas permit a person to visit the U.S. for a finite duration for a specific purpose, and does not confer the right to live or work permanently.

Some visas are for immigration purposes, meaning the bearer may use them to enter the U.S. to stay indefinately.

The U.S. government permits citizens of certain countries to travel to the U.S. without the need for a visa.  These countries fall under the Visa Waiver Program (VWP) administered by the Department of State.  For more information on this program, including participating countries, click here:  http://travel.state.gov/visa/temp/without/without_1990.html#countries.