Articles that are reviewed or critiqued by the author’s peers (other experts in the field). Peer-review is a quality control process. It’s done to improve the article & make sure it is of sufficient quality to be published in a scholarly journal.
Many of our databases have a way to check a box to limit to scholarly, peer-reviewed journals when searching. It's not always a fool-proof method, so you'll have to evaluate the specific article using the criteria discussed on this page.
The library database search results often help you to identify the type of article you have found. In the EBSCOhost databases (Academic Search Complete, SocINDEX with Full Text, etc.), it will label peer-reviewed journals as Academic Journals (as opposed to News or Periodicals).
A chart comparing the various sources.