Scholarly journal articles are those that are typically:

• Peer-reviewed by experts in the field.
• Published in reputable journals from scholarly or professional societies, university or academic presses, or other research organizations (such as government or nonprofit agencies).
• Indexed in one or more research databases that focus on particular subject areas.

Other types of articles may appear in trade publications, popular magazines, newspapers, online scholarly blogs, or digital research commons. Most of these are not peer-reviewed, but may contain useful information or citations. Many of the library databases include these types of publications, but results can be filtered to identify peer-reviewed publications only.

"Primary sources" refers to the original source material that you are studying, which may include observations, data, experimental results, interviews, narratives, case studies, or other original research materials.  Primary source materials may include quantitative or qualitative data, and may be derived from clinical, experimental, or field research. The term, "Primary sources" can also be used to describe documents, images or artifacts that provide firsthand testimony or direct evidence concerning an historical topic under research investigation. Primary sources are original documents created or experienced contemporaneously with the event being researched and can include photographs, audio recordings, video recordings, journals, letters, diaries, and more.

Most journal articles and book chapters are "secondary sources" that offer analysis, interpretation, or critical responses to the original object of study, including those primary sources of data and observation. They are meant to synthesize, organize, and explain the information found in primary sources.

An "empirical" study is one that is based on the original creation or gathering of data & evidence, and a subsequent analysis & interpretation of that material. It generally precedes those studies that summarize, synthesize, or organize previous research into a broader analysis.

A "literature review" is an article that synthesizes a number of other secondary sources and provides an overview of research on a subject. Many empirical studies include a brief literature review, but longer reviews are often published as a separate article or book chapter. In a database search, you can specify "literature review" in your search terms to identify these articles.

See also the UNT Libraries Literature Review Process guide for more information on conducting literature reviews. 

A "systematic review" is a more extensive, structured analysis of a large body of scientific literature on a particular topic. It should include a comprehensive, replicable, and fully-documented search methodology that aims to identify all of the relevant literature on the topic, organize it, and evaluate it according to the researcher's established criteria.