1. They are written by academics and professors, and aimed at fellow scholars in the field
But this does not mean that undergraduates or non-academics cannot use them, only that they will be professional in nature and in accordance with the professional standards of that discipline -- a good thing for you!
2. The language is scholarly and/or technical
3. The articles include extensive documentation and bibliographies
4. Published by scholarly or university presses
5. They often do not exist to make money. Their goal is not monetary, rather it is to contribute to that specific field.
From: University of South Alabama - University Library Peer Review: What is It? Libguide
From Cornell University Olin Library's "Research Minutes" series:
Peer-Reviewed materials are publications that have been reviewed by expert readers prior to their publication. Also called "Refereed" materials, these sources have been evaluated by experts in that particular discipline for accuracy, merit, methodology, and overall contribution to that specific discipline. These reviewers will alert the publisher of the material to any mistakes etc. that should be corrected prior to publication. This review process provides that only solid, well-done, and timely articles will be included in these publications. Generally, this is why your professor will encourage you to use these sources.
Note that "scholarly” and “peer-reviewed” are related but not the same: not all scholarly sources are necessarily peer-reviewed, but most peer-reviewed articles would be considered scholarly.
Confused? If so, ask your professor or your friendly reference librarian.
From: University of South Alabama - University Library Peer Review: What is It? Libguide
