The North American Free Trade Agreement (NAFTA) was created with the purpose of permits qualified Canadian and Mexican citizens to temporary engage in business activities at a professional level. 

There is certain type of professionals who are eligible as TN nonimmigrants. Accountants, engineers, lawyers, pharmacists, scientists, and teachers are one of them. 

Among other things, one of the criteria to identify if you are eligible has been able to prearrange a full or part time job with a U.S. employer but not self-employment. 

The main difference between in this category is that Canadian citizens do not need to apply for a visa to gain entry in the United States. They just would need to establish eligibility for TN classification at the time they seek admission by presenting required documentation to a U.S. Customs and Border Protection (CBP) while Mexican citizens are required to obtain a visa to enter the United States as a TN nonimmigrant. The process of obtaining a visa begins by applying directly at a U.S. embassy or consulate in Mexico.  

For both nationals the initial period of stay is three years with an option of extending it by seeking an extension of stay. Also, any accompanying or “following to join” spouse and children under the age of 21 may be eligible for TD nonimmigrant status.

Elliott Immigration Law will help you to determine if you are eligible and to know more specifics about the process.