The Immigration and Naturalization Service (INS) is a unit of the U.S. Government, Department of Justice, and enforces the immigration laws.
Generally the American Embassy or Consulate in your home country will give you a visa, stamped into your passport, which allows you to enter the U.S. Under the Visa Waiver Pilot Program, tourists from many countries (such as Canada, Japan, New Zealand and most European countries) with a valid passport are allowed to travel in the U.S. without a visa. Such tourists are allowed to stay up to 90 days.
The INS is supposed to keep out aliens who have a criminal history, such as those who have been convicted for crimes such as murder, robbery, rape, forgery, burglary, welfare fraud, tax evasion and drug related problems. Similarly, aliens who are involved in terrorist or sabotage activities are not admitted, nor are aliens who suffer from certain illnesses or communicable diseases. Aliens who are not able to finance themselves because they don't have sufficient income or resources and could need public assistance are not allowed to enter. The INS also excludes people who obviously are lying about their intentions for coming to the U.S. Commonly visas are denied because of the failure of the applicant to prove that he/she have ties abroad that would compel them to leave
the U.S. at the end of the temporary stay (often called the 214(b) visa refusal).