What is the difference between a journal, a periodical, a magazine, a newspaper, and a trade journal? Is there a difference between an academic journal, a peer-reviewed journal, and a scholarly journal?
A periodical is anything that comes out periodically. Magazines, newspapers, and journals are all periodicals. They may come out daily, weekly, monthly, quarterly or annually, but new issues are released on a fixed schedule.
Magazines, newspapers, trade journals, and academic journals are intended for different audiences.
Magazines are for the general reader. Magazines typically have advertisements for popular consumer products like cars, perfume, or electronics.
Newspapers are also for the general reader. Both newspaper and magazine articles are written by journalists. While newspapers may be focused on authoritative and quality news reporting, journalists do not have to disclose their sources.
Trade journals are specifically created for particular professions, or trades. They also have advertisements directed to that profession. For example, a trade journal for dentists may advertise and discuss new dental technologies.
Academic journals, peer-reviewed journals, and scholarly journals often are synonymous. Most scholarship is produced by professors and graduate students, and also by experts outside of academia. Trade journals and magazines typically depend on editors to determine what gets published and what doesn't, while scholarly journals depend on the peer review process.
Peer-reviewed: When a scholar produces research and writes a paper/article about it he or she sends it to a scholarly journal. The editor of that journal removes the name from that research article and sends it to experts in the field to review. Since the person submitting the paper or article is also presumed to be an expert, these other experts are his or her peers. Hence the name 'peer-reviewed.' These peers read the research article and then recommend to the editor whether the article contributes significantly to the body of knowledge. If they approve of the article then it gets published. If they do not then it gets rejected.
The process is kept anonymous so that articles are not accepted or rejected based on personal likes or dislikes. The peer reviewers are to focus solely on the quality of the research and argument presented in the article submitted for publication.
The peer review process isn't perfect, but it's the best way scientists, scholars, and other researchers have developed to ensure high-quality information. This is why your professors often ask you to focus on peer-reviewed literature. This is typically the most rigorous and highest quality research available.
Here is a handout which may help you distinguish academic/scholarly journals from popular magazines.