Report original research by the author(s). Also known as empirical studies. You will usually see terms such as "experiment," "empirical," "research," or "study" in the title, abstract, or introduction of the article. These articles usually are organized into sections, which describe methods, study participants, and results. Considered primary sources.
Critical evaluation of existing literature on a certain topic or field. These include literature reviews. These articles can provide you with a good overall understanding of a given topic. Considered secondary sources.