Visas and Travel
A visa is like a "ticket of admission." You apply for it outside the U.S. and need it to enter the U.S. Those of you coming temporarily to study in the U.S. usually have a non-immigrant student or F-1 visa. Your visa type, expiration date, issuing office, and the number of entries permitted will be stamped in your passport. You may stay in the U. S. even if your visa has expired, but you may not leave and then re-enter if your visa has expired. (An exception occurs if you are returning from an adjacent country, e.g., Canada or Mexico, after a stay of less than 30 days, and you may then be eligible for an automatic extension at the border.) Note that Canadian students need an I-20 form but do not need to have an F-1 visa stamped in the passport.
The letters "D/S" that may be written on your form stand for "Duration of Status" and indicate that you may stay in the U. S. as long as you are a regularly registered, full-time student. Different visas have different advantages and restrictions. Check with the Office of International Student Services for information regarding your visa.
If you have an F-1 visa, for reentry you will need your passport with a current visa stamp showing that multiple entries are permitted, and an I-20 form endorsed by the Office of International Student Services. Please do not misplace your I-20 forms.
For other kinds of visas, other documentation may be needed. Permanent residents, for example, need to show their "green card" in addition to the passport.
Please note that Canada requires a visa for admission of many international students, depending on their country of citizenship.