Citizens of foreign countries can travel to the U.S. for different purposes. They can get immigrant visas or non-immigrant visas based on the purpose of their travel. Immigrant visas are meant for those who wish to live in the U.S. permanently. Whereas non-immigrant visas are for temporary visitors.
People who wish to visit the U.S. for tourism or for pleasure, international students who want to study in the U.S., people who want to undergo medical treatment and foreign workers who wish to work in the U.S. on temporary basis can get non-immigrant visas.
Temporary non-immigrant visas will be valid only for a certain period of time. Non-immigrants need to leave the U.S. before their authorized period of stay expires. It must be noted that the date of expiration of the temporary visa and the date of expiration of the authorized period of stay are not the same.
Non-immigrants entering the U.S. will be issued Form I-94, Arrival/Departure Records. At the U.S. port of entry, immigration authorities will determine how long the non-immigrants can stay in the country on non-immigrant visas. The date on which they must depart the U.S. will be stamped on their I-94 cards this is the date on which their authorized period of stay will expire. They must leave the country on that date. People who stay longer will be subject to deportation. However, they can stay back in the U.S. if they have been granted an extension of stay.
A visa will only permit foreign nationals to travel to U.S. ports of entry. At the port of entry they need to request permission to enter the U.S. Immigration authorities at the ports of entry alone have the authority to decide whether or not to permit the non-immigrant visa holders to enter into the U.S. If they determine that the non-immigrant visa holders are admissible into the country, they will stamp their travel documents and permit them to enter the U.S. In case they are deemed inadmissible, they will be required to travel back to their home countries or subject to deportation.