An entry VISA is a travel document issued by a U.S. Consulate and attached to a page in your passport that is evidence of your eligibility to be admitted to the United States in a particular status with supporting documents. After admission to a program of study, International students who require an F-1 student visa to study in the U.S. apply for a certificate of eligibility called an I-20 from the International Students and Scholars office (ISSO). The I-20 is needed to apply for the entry visa to present upon arrival.
The two most important requirements are to remain registered full-time every term, making normal progress toward your degree, and not to accept any employment, either on- or off-campus, without written permission from the ISSO, and, if necessary, authorization from the Department of Homeland Security (DHS). Be familiar with the information in Maintaining F-1 status. Under certain conditions status may be reinstated if lost for reasons beyond your control. You may also regain status by leaving the U.S.