The TN visa is a temporary visa available to certain citizens of Mexico and Canada. The application can be done at the local consulate in either Mexico or Canada if the intended employee is not already in the United States. If the employee is in the United States, then a change of status form can be filed with USCIS to allow the employee to remain in the United States without having to leave.
To qualify, the employee must have a letter from a prospective employer in the field that matches their educational background. In addition, the position must be included in an approved list from the U.S. government. A list of qualifying jobs can be found at this link: http://www.nafsa.org/_/file/_/amresource/8cfr2146.htm
The TN visa is initially valid for up to 3 years as long as the employee continues to work in a qualified field. The employee is able to extend their TN visa indefinitely as long as they continue to work at a qualified job.
Dependents of a TN visa holder can also come to the United States, but will not be permitted to work.