A scholarly source is a written work that has been peer-reviewed, or refereed. The terms peer-reviewed and refereed are interchangeable, and refer to a process of evaluation. Experts (peers) in the subject matter evaluate (review or referee) the work, and deem it deserving or undeserving of publication. The evaluation measures the authority, accuracy, and relevancy of the work to ensure high quality. Scholarly sources are widely considered the most reliable sources of information.
Example: The Journal of the American Medical Association
Popular sources provide information on various topics, current events, or popular culture. These sources are authored by journalists or professional writers and aim to inform or entertain a general audience. This type of publication might be edited for spelling, grammar, organization, etc., but thy are not peer-reviewed.
Example: TIME Magazine
Report original ideas, events, and scientific studies.
Example: Journal article reporting results of a clinical trial.
Review and restate available information on a topic.
Example: Published literature review.
Synthesize information on a topic - this type of source usually provides a broad overview of a topic.