Popular sources: are widely available, usually cheaper to acquire, and can be understood by almost every person with basic literacy skills. They tend to promulgate known ideas and theories. These works may be professionally edited, but do not go through a jury process.
Scholarly or Academic sources: The purpose of Scholarly or Academic sources is to share information within the subject field and they are based on original research and experimentation. They are suitable for academics, and are supported by a system of learning and study. They are less widely circulated than popular sources and may be understandable only to those who work or study in a particular field.
In addition, scholarly sources are juried either through peer review or the referee process to determine that the research meets a standard of accuracy, originality, and scholarly integrity.
Peer-Reviewed means that an has been evaluated by a group of subject experts prior to being published in a scholarly journal. When an article is Peer-Reviewed, the editors of the journal wishing to publish the item send it to scholars in the relevant field; e.g., an article about Biology would go to other biologists. These scholars provide feedback about the article's pertinence to scholarship in their field, the quality of research and presentation of findings, and more. This ensures that the articles that wind up in academic journals have scholastic merit and contribute to the overall research in the field.
Note: Not every article published in a scholarly journal is peer-reviewed. Book reviews, letters to the editor, etc. are not peer-reviewed.
Refereed: A Refereed Article is also referred to other scholars in the field. However, in this instance, the reviews are blind. In other words, the academic peers conducting the review do not know the name of the work's author. In addition, it is often the case that the reviewers' names are not made known to the author. This ensures that the work is judged solely on its own merit rather than the author's reputation. In addition, the manuscript must be reviewed by at least two other people.