Popular sources include news articles, magazine articles, most web sites, and most videos. They may provide current or even up-to-the-minute information. Although some have been carefully checked for accuracy, others have not.
Scholarly sources include articles in peer-reviewed journals, peer-reviewed books and book chapters, and most reports issued by universities, reputable government agencies, and research organizations such as RAND. Most scholarly sources "look scholarly," and many use scientific terminology or statistical techniques that may not be immediately understandable to non-specialists. Peer review -- careful, anonymous evaluation by two or more subject experts -- helps ensure quality. However, peer review can also delay publication by months or even years, and it is generally not designed to detect fraudulent or intentionally misleading data and results.
Scholarly sources are generally preferred for academic coursework, but reputable popular sources are sometimes better at presenting current information or providing clear overviews of complex topics.
Books from university presses are peer reviewed. Other books may or may not be peer reviewed, and the "popular vs. scholarly" distinction is often more clear-cut for articles than for books.
Your decision to use or not use a particular information resource should be based on more than the quality of the item itself. In particular, you should consider whether (and how) the resource is relevant to your own needs. In assessing relevance, you may want to consider
The significance of each factor will vary with the goals of the assignment.
Some scholarly works (articles, books, etc.) are far more important than others. For instance, journal articles in the social sciences are never cited in the published literature while others are cited hundreds or even thousands of times.
For scholarly articles, peer-reviewed status is not a guarantee of quality, but simply the minimum standard that sets the work apart from popular (non-scholarly) writing. Ideally, you want to make use of the best literature—the most important findings, the most innovative ideas, and the studies that have had the biggest impact on subsequent research.
Google Scholar can help you assess the scholarly impact of each article or document, since it includes a notation, “Cited by ##,” next to each search result. That notation indicates the number of times the work has been cited in subsequent research. Citation counts can also be found within Scopus.