The TN visa is a special category visa and status specifically designed for Canadians and Mexicans who work in a professional capacity in countries that are signatories of NAFTA. This visa/status is for legal work in the United States and Canada, and it lets Canadian and Mexican citizens work where they are needed in certain occupations.
While it is quite similar to an H1B visa, the TN visa does have unique features of its own. This status lets a US citizen, Mexican or Canadian work for up to 3 years, and it may be renewed indefinitely in increments of 3 years. Despite the ability to incrementally renew, it is not considered to be a permanent visa. If immigrant officials think that it is being used as a green card, they have the discretion to deny any further renewals.
Spouses and/or children of workers also usually receive TD status, so they may be admitted to the US with the TN status/visa holder. Although the TN status is often called a TN visa, it is not a visa, because Canadians do not formally have to request a TN visa at a consulate.