The TN Visa is very similar to the H-1B visa in that it is a temporary work visa for professionals. The TN visa was created under the North American Free Trade Agreement (NAFTA), and so it is available exclusively to nationals of Canada and Mexico. The regulations define who qualifies as a “professional.” Some professionals that qualify are accountants, engineers, lawyers, pharmacists, scientists, and teachers.
As a Canadian, you must present certain documentation at the border in order to enter in TN status. This documentation includes evidence of your Canadian citizenship and evidence that you are a “professional” as defined by the regulations. If you are Mexican, you must submit an application to a U.S. consul before you can be admitted. Applications for Mexican nationals must include a certified Labor Condition Application and are subject to prevailing wage requirements and attestations.
There is no fixed duration for which you may remain in the U.S. in TN status, but it must be evident that your stay is in fact temporary.