What is a Visa?
A U.S. non-immigrant visa is issued in an individual's passport by a U.S. consul in a U.S. embassy or consulate outside the United States. A visa is used solely for the purpose of entry and reentry to the United States. The visa does not determine how long an individual may remain in the United States. Therefore, an individual may stay beyond the expiration date of a visa as long as the visa document (Form I-20, Form DS-2019, or Form I-797) is valid. Most visa stamps are for multiple entries. It is not possible to apply for or to renew a visa in the United States.
Canadian citizens are not required to obtain U.S. visas in their passports for entry into the United States. Instead of a visa, Canadian citizens are currently admitted to the United States in a particular visa status by presenting their valid Canadian passports and the appropriate visa document for their visa status e.g. Form I-20, Form DS-2019 etc.