Generally...

Visa allows a foreign national to enter the United States.  Visa for short or temporary stay in U.S. is generally called nonimmigrant visa.  A nonimmigrant visa holder returns to his country upon completion of the purpose.

There are many types of visa based on purpose of U.S. entry.  Visa applicant must have a purpose that aligns with a visa category.  Visa is granted if the Consular officer is convinced that stated purpose meets a visa category.

Entry into the United States is granted only when approval is granted by the Customs and Border Protection (CBP) officer i.e. grant of visa is not an assurance that the foreign national will be admitted to the United States.

Parole

Parole is not a visa but a special permission to enter United States temporarily (up to a year) for a specific purpose. 

It is a discretionary grant for urgent humanitarian reason or for public benefit.  International entrepreneurs can now use parole to invest in and grow startup businesses.  USCIS would soon form policies for IER parole.

Temporary Protected Status (TPS)

Nationals of designated countries in United States qualify for this lawful stay.  Country designation is made by the U.S. government from time to time and this status expires when the designation is withdrawn.  Example: Liberia is no longer a TPS designated country.

B: Visitor for business or pleasure

Tourists, short term visitor for business purpose including conferences, business meeting, training and business promotion.

F/M: International (full-time) Students/ Vocational Students

Study in an educational institution that is recognized to accept international students.

H1B Professionals: Specialty Occupation

professionals whose position requires a bachelor or masters diploma or higher qualification. 

K1: Finace` Visa

This is a hybrid category between immigrant and nonimmigrant visa.  Unmarried foreign nationals in relationship with a United States Citizen coming to marry the fiance`.

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