The I-20 is the official government form that certifies that you are eligible for non-immigrant student status under an F-1 or M-1 visa. The form is standard for every SEVP-certified institution and must be processed by the school.
F students will receive the I-20 for academic and language students, while M students' is for vocational students. Along with your passport and visa, you must bring the I-20 with you, with a valid travel signature, every time you re-enter the United States. The travel signature must be renewed every year while you complete your studies and every six months if you are on OPT.
For more information about the I-20 form, click here.