Many instructors require the use of scholarly articles in research papers. Scholarly articles are found in peer-reviewed, or refereed, journals. Such journals have editorial boards of experts who accept or reject articles for publication. Therefore, the articles are considered high quality and represent important research in a given field.
Articles from popular magazines do not include the same level of research and are not reviewed in the same way. They often contain stories of interest to a wider audience.
In addition to popular and scholarly sources, you will find trade journals (practical information for professionals) and journals of opinion (information expressing a given viewpoint).