Because of their short length, articles often exclude background info and explanations, so they're usually the last stop in your research process, after you've narrowed down your topic and need to find very specific information.
The main thing to remember about articles is that they're almost always published in some larger work, like a journal, a newspaper, or an anthology. It's those "article containers" that define the types of articles, how you use them, and how you find them.
Articles are also the main reason we have so many databases. The Library Catalog lists everything we own, but only at the level of whole books and journals. It will tell you we have the New York Times, and for what dates, but it doesn't know what articles are in it. There is no single database that indexes all articles.
While newer journals and magazines are usually online, many older issues are still only available in paper. In addition, many of our online subscriptions explicitly don't include the latest material, specifically to encourage sales of print subscriptions. Older newspapers are usually transferred to microfilm.
The terms academic or scholarly journal are usually synonymous with peer-reviewed, but check the journal's publishing policies to be sure. Trade journals, magazines, and newspapers are rarely peer-reviewed.
Primary or Secondary Sources
In the social sciences and humanities, articles are usually secondary sources; the exceptions are articles reporting original research findings from field studies. Primary source articles are more common in the physical and life sciences, where many articles are reporting primary research results from experiments, case studies, and clinical trials.