The TN is a treaty-based visa category for Canadian and Mexican professionals. The U.S. job must be among those listed in the North American Free Trade Agreement (NAFTA), and the foreign national must have the NAFTA-defined qualifications for that position (usually, but not always, a bachelor’s degree).
Mexican citizens must apply for a TN visa at a Consulate, but Canadian citizens can apply directly at the border and do not need a visa.
The TN category allows for an initial 3 year stay, which may be extended.