Visas are obtained from U.S. consuls abroad. It certifies the visa holder has been examined and is permitted to seek permission to the US.
There are two types of visas: immigrant and nonimmigrant visas.
Immigrant visas are issued after you have established eligibility. They are for permanent residency. It has a six-month validity period during which time the immigrant must apply for admission.
To get an immigrant visa, the foreign citizen must be sponsored by a U.S. citizen relative, a U.S. lawful permanent resident (LPR), or by a prospective employer. The person also has to be the “beneficiary” of an approved petition.
Nonimmigrant visas are for international travelers or business people coming to the U.S. temporarily. Common types of nonimmigrant visas are: Business visitors (B-1), Tourism, vacation, pleasure visitors (B-2) Students (F).
People from certain countries can enter the U.S. for up to 90 days as a visitor for business or pleasure without first getting a visa. This is called the visa waiver program (VWP). However, if you enter on the VWP there are no extensions or change of status.