Peer-reviewed papers are scrutinized by experts to ensure that they meet the discipline's high standards. Peer-reviewed papers are also called "scholarly articles" or "refereed articles."
Image courtesy University of California Museum of Paleontology’s Understanding Science
Peer-reviewed journal articles are published in peer-reviewed journals. The easiest way to make sure that you're searching for peer-reviewed journals is to check the "peer-reviewed" box which appear in most databases.
If you're not sure if a journal is peer-reviewed, check in Ulrich's. Ulrich's Periodicals Directory is a database that maintains information about all types of periodicals: scholarly journals, newspapers, magazines and more. If you search your journal title in Ulrich's and you see a mini referree jersey (see image below) in the search results, your journal is peer reviewed!