What is the difference between a popular source, such as a magazine, and a peer-reviewed source/journal?
Magazines are written for the general public. They often are full of advertisements.
Journals, on the other hand, are targeted to students or professionals working in a particular field. They usually have very few ads in them. The articles include bibliographies at the end, and the author's or authors' credentials (where they went to college and where they work) are given.
Sometimes a publication is peer-reviewed, which means that all the articles have been read and approved for publication by experts in the field.
Peer-reviewed materials are excellent sources of information for scholarly papers!
Peer-reviewed = Articles read & approved for publication by experts in the field
Experts = People with advanced degrees in the subject or who have worked in the field for many years