Popular sources are typically written by journalists or staff writers, who are employed by the publisher for which they write. Popular information sources cover news and current events in a field; report on news of general interest; write profiles of people, places, or events; and express political opinions. These types of sources are typically full of advertisements, and are focused on making a profit.
Trade sources are typically written by people working a particular field, for other people working in that field. They typically discuss practical application; cover news in the field; present brief reports on research; offer opinions about trends, events, and industry/forecasts. They are a good source of information if you are looking for news and trends about a particular field or occupation.
Scholarly sources, usually articles, are written by researchers who are experts in their field. Often, these experts are employed by colleges, universities, scientific or medical laboratories, or other institutions of education or research. Often, the goal of a scholarly article is to present new and original research conducted by the authors.
Scholarly sources are considered much more credible than other sources because of peer review.
The most-respected scholarly journals are peer-reviewed, which means that other experts in their field check out each article before it can be published. It’s their responsibility to help guarantee that new material is presented in the context of what is already known, that the methods the researcher used are the right ones, and that the articles contribute to the field.
Peer-reviewed articles are more likely to be credible. Peer-reviewed journal articles are the official scholarly record, which means that if it’s an important development in research, it will probably turn up in a journal article eventually.
When talking about "scholarly" articles, most professors usually mean that they want you to find a peer-reviewed source.
Examples: TIME, Teen Vogue, WIRED
Examples: Today's Veterinary Technician, ABA Journal, Progressive Grocer
Examples: Cell, American Journal of Epidemiology
