What is a visa?

The F-1 or J-1 visa stamp in your passport is permission to apply to enter the United States in that visa category. Canadian citizens are not required to have a visa stamp in the passport to enter the U.S.

The F-1 and J-1 category of visa will allow you to enroll in credit bearing classes at a Student and Exchange Visitor Program approved school in the U.S.  Visitors who arrive to the U.S. under other visa classifications may not have approval to study.   You can refer to this chart to view other classifications and the ability to study under those classifications 

It is okay to remain in the U.S. after your F-1 or J-1 visa expires as long as your passport and I-20 or DS-2019 are valid, and you are a continuing full-time student. The visa expiration date is separate from your length of authorized stay in the U.S.

If your visa expires while you are in the U.S., the next time you travel internationally you must apply for a new F-1 or J-1 visa in order to return to the U.S. This is also true if your visa has a limited number of entries that have been used, or if you have changed your nonimmigrant status while in the U.S.