Employment-Based Visa Petitions
If your stay will be temporary, then you'll want to make an application for a non-immigrant visa. This type of visa allows you to travel to a U.S. port-of-entry to request admittance from a Department of Homeland Security official vs an immigrant visa that may be issued to a foreign national who intends to live and work permanently in the United States.
For example, if you are coming to the U.S. as a temporary visitor for pleasure, under U.S. law you would apply for a "B-2 Visa". If granted, you may come to the U.S. for a temporary period of time and you may only engage in activities consistent with being a visitor, such as sightseeing, visiting friends and relatives. You would not be permitted to enroll in school and study nor would you be allowed to work, as that is not consistent with a B-2 visa. If you are in the U.S. on a Tourist Visa, it is possible to change status to a different category, such as student, assuming you qualify.
Immigrant visas are for aliens who are entering the U.S. with the intention to make the U.S. their permanent residence. Immigrant visas are subject to numerical limitations. That means that each of the immigrant visa categories has a certain number of allotted slots for visas. Once the quota is filled, a "waiting list" is created that will delay your arrival into the U.S.