Scholarly sources, or academic sources, can often be characterized by the following features:
Scholarly, peer-reviewed journal articles are the most appropriate sources to use in academic papers and projects, and they will be the best sources for primary research.
A scholarly, peer-reviewed article is one that is published only after undergoing scrutiny by several scholars, called reviewers, in the author's discipline, e.g., chemistry, history, etc. The reviewers do not know the identity of the submitting author and the author, likewise, does not know the identity of the reviewers. This method insures that the results of the peer-review process are fair and impartial. The reviewers may choose to reject the article for publication or recommend that the article be published either with or without suggested changes.
Popular, non-peer-reviewed articles are ones that do not undergo academic scrutiny; these kinds of articles are generally found in news magazines like Time and National Geographic.
View this short video from NCSU libraries that explains the peer review process.