There are three distinct types of articles: scholarly, popular, and trade.
Scholarly articles are usually lengthy and written in scholarly language by scholars and researchers. These articles are often peer-reviewed, which means that the information in the article was checked for accuracy and importance by a team of experts in the field before publication.
Scholarly articles often report on research. Research articles can be identified by their standard format, containing sections titled Abstract, Methods, Results, Conclusions, and Bibliography.
Be aware that scholarly journals sometimes contain non-scholarly content, such as book reviews or editorial opinions. While these articles can be useful they do not qualify as "scholarly" or "peer-reviewed".
If you are not sure whether an article is peer-reviewed, you can look up the journal in the Ulrich's Periodicals Directory to find out, or check the author guidelines on the journal's web site.
Popular articles are the articles in magazines that you encounter in the grocery store or the dentist's office. The articles are written by journalists to inform and entertain, and frequently give short, easy-to-read overviews of the research articles published in scholarly journals.
Trade journal articles are a third type of journal article you might encounter. These articles contain practical advice and information for workers within a specific occupation. For example, a professional pharmacist might read the trade journal Pharmacy Times to find out about new advances in child-proof lids, but she will turn to scholarly research articles to learn how a new drug performed in clinical trials.