Along with being a primary source, it is frequently important that you know if your source is scholarly and appropriate for academic research. Some traits of scholarly articles are...
Citations to work done by others
Language is often serious and technical
Images are usually charts, graphs, or otherwise informative, rather than glossy photographs or advertisments
Authors' names are given, along with their affilitions with university, research institutions, etc.
Date of publication is given, frequently along with the date on which the articles was submitted for peer review
"About" or "instructions for authors" link on the journal's Web site indicates that the journal is peer reviewed or describes its peer review process
Peer Reviewed Journals publish scholarly articles. Some library databases let you limit your search results to peer reviewed journals, but they don't completely screen out content that is not peer reviewed. Pay attention to the other signs that a source is scholarly.