A visa is a document issued by a sovereign government giving permission for a foreign national to approach a portal of entry, such as an airport, and request entrance to its borders for a limited period of time and for a specific purpose. Visas can either be attached to a passport or be issued as a separate paper, but both documents are usually required to enter a foreign country. Some friendly nations allow travel amongst themselves without visas, while others require an exit visa for their citizens to leave at all in the first place.
There are three basic types of visas for entering the United States. Visitations visas are granted to individuals planning a limited stay, whether for business or pleasure. Student visas are available for limited numbers of eligible students looking to study in the country. The requirements for obtaining a non-immigrant work visas depend on the type of profession involved, and may require certification from the Department of Labor or other government agency before being granted.
A few general requirements apply to all visa applications. Among them are evidence of compelling social and economic ties abroad, as well as a residence, all of which tends to ensure the individual will leave the United States at the expiration of their visa. The applicant must state a purpose for their visit--business, pleasure or medical treatment--and must show evidence of funds to cover reasonable expenses on their trip. The minimum required paperwork includes the Non-immigrant Visa Application (DS-156), a Supplemental Non-immigrant Visa Application (DS-157), which provides more detailed information about travel plans, a valid passport and a 2-by-2 inch photograph.
Visa requirements serve two main purposes. First, the visa requirements prevent illegal immigration by putting the burden on the applicant to demonstrate that their visit to the United States is temporary. Those looking to stay on a more permanent basis need to file for a green card, and are subject to immigrant quotas. Second, the visa process documents all foreign travelers entering the United States, providing information that can be crucial to national security efforts.
Citizens of 34 countries are eligible to travel within the United States for 90 days or less without obtaining a visa under the Visa Waiver Program (VWP). The program started in 1986 to stimulate tourism and relieve some of the burden from the Department of State. The list of countries participating in the VWP are primarily America's European allies, and also includes the Czech Republic, Estonia, Hungary, Korea, Latvia, Lithuania and Slovakia. Other countries on the list are Australia, Japan, the U.K., Ireland and Iceland. Travelers from these countries need only machine-readable passports to make limited journeys to the United States.
Applying for a visa is done through the nearest U.S. consulate or embassy. The process can take several weeks, depending on the location, and involves a personal interview. Acquiring the visa itself is not a guarantee of entry into the United States. It is a prerequisite, however, for traveling to a port-of-entry and requesting permission to enter. The ultimate decision is made by the Department of Homeland Security and the Customs Border Protection immigration officer on duty.