Answer
Primary research articles (also called original research or empirical) are primary sources. Such articles describe newly done (though not necessarily unique) experiments executed with the objective of generating new evidence on a particular topic. All primary research articles taken together are often referred to by scientists as "the literature."
Components of a primary research article include:
- Abstract - brief description of the why and how of the research, as well as the results and main conclusions
- Introduction - reviews the pertinent literature as background for the research subsequently described
- Materials and Methods - how the research was done with enough specifics so it can be reproduced
- Results - what was found, often supported with figures and tables
- Discussion - what the results suggest or establish, how this research fits in with previous research, potential usefulness of this research, etc.
- References - sources used
Review articles (also called literature reviews) are secondary sources. Such articles provide an overview and discussion of primary research done on a particular topic, and can point out problems and suggest needed additional research. Review articles help experts keep up to date in their fields.
Review articles often contain some of the same sections as primary research articles, such as an abstract, introduction, and references. The most important difference you'll see when you read a review article is that it is about research previously done, and therefore is a secondary source.