In short, an immigrant visa is issued to people that have been given the right to work and live permanently in the United States while a non-immigrant visa is issued to people temporarily visiting the U.S. for tourism, medical treatment, temporary work, school and other reasons.
The two are very different.
Immigrant status is given to people that have been granted the privilege to live in the U.S. permanently. Synonymous terms for immigrant status are: permanent resident, immigrant, green card holder and resident alien. To acquire immigrant status, most people are sponsored by a family member or employer in the United States. Other people may become permanent residents through refugee or asylee status or other humanitarian programs. Depending on the path, gaining immigrant status can be a lengthy and complex process.
Non-immigrant status is for people who wish to enter the U.S. as a temporary visitor. The purpose for the visit may be tourism, business, temporary work, school or various other reasons. Once a person has entered the U.S. in non-immigrant status, they are restricted to the activity or reason for which they were allowed entry. Most non-immigrant visas are issued only to applicants who can demonstrate their intentions to return to their home country.
People applying for a non-immigrant visa may be denied if they appear to have the “dual intention” of visiting the U.S. temporarily while pursuing permanent resident status. Presently, only E, H-1 and L non-immigrant categories are allowed to enter and remain non-immigrants while simultaneously pursuing permanent resident status. When applying for a non-immigrant visa, it is important that the applicant be able to demonstrate family and employment related ties to their country of origin. Without these ties, the visa officer from a U.S. consulate may conclude that the visa application is only a pretext for an intent to stay permanently U.S. and may deny the application.