"Peer reviewed" (or "refereed" journals) are those that subject content to a critical review by other experts in the field prior to accepting a manuscript for publication. Thus, limiting your literature search to the peer-reviewed journals ensures a higher level of scholarship and research methodology. Magazines, trade journals, and newspapers tend to be "non-peer-reviewed," meaning perhaps just the editor or someone who is not an expert in the field has reviewed the content before publishing. How do you know if a journal citation is from a peer-reviewed journal?
Google Scholar is a time-saving, scholarly search interface accessible from within the Google interface. With Google Scholar, you can access peer-reviewed journal articles, books and book sections. For literature searching, specialized databases have more functionality and access more comprehensive results, but Google Scholar is a good tool