What is a Visa?

From Travel.State.Gov (http://travel.state.gov/visa/questions/what/what_4429.html)

A citizen of a foreign country who seeks to enter the United States (U.S.) generally must first obtain a U.S. visa, which is placed in the traveler’s passport, a travel document issued by the traveler’s country of citizenship.  Having a U.S. visa allows one to travel to a port of entry, airport or land border crossing, and request permission of the Department of Homeland Security (DHS), Customs and Border Protection (CBP) inspector to enter the U.S.  While having a visa does not guarantee entry to the U.S., it does indicate a consular officer at a U.S. Embassy or Consulate abroad has determined you are eligible to seek entry for a specific purpose.

The type of visa one must obtain is defined by U.S. immigration law, and relates to the purpose of travel. There are two main categories of U.S. visas:

• Nonimmigrant visas – For travel to the U.S. on a temporary basis.
  
  
• Nonimmigrant visas are for international travelers (citizens of other countries), coming to the U.S. temporarily.
• The type of visa needed is defined by immigration law, and relates to the principal purpose of one’s travel.
• Most Canadian citizens and many citizens from Visa Waiver Program countries (See Appendix 4.1) can come to the United States without a visa if they meet certain requirements. All Visa Waiver Program travelers must present a machine-readable passport at the U.S. port of entry to enter the U.S. without a visa; otherwise a U.S. visa is required.
• Immigrant visas – For travel to live permanently in the U.S.
  
  
• In general, to apply for an immigrant visa, a foreign citizen must be sponsored by a U.S. citizen relative(s), U.S. lawful permanent resident, or by a prospective employer, and be the beneficiary of an approved petition.
• The first step to become a permanent U.S. citizen is to file a petition with U.S. Citizenship and Immigration Services (USCIS)
• In general, petitions to become a permanent U.S. Citizen must be filed in the United States.  U.S. citizens and lawful permanent resident sponsors residing in the United States file with the USCIS Chicago Lockbox facility or Phoenix Lockbox facility, following instructions on the USCIS website. U.S. employers file as instructed on the USCIS website.
• Under certain circumstances, some petitions may be filed outside of the United States.

 As of July, 2013 visas appear in this format: