In general, secondary sources are accounts written after the fact with the benefit of hindsight. They are interpretations and evaluations of primary sources. Secondary sources are not evidence per se, but rather, commentary on and discussion of evidence.
To do research, you must cite to research. Primary sources do not represent research per se, but only the artifacts from which most research is derived. Therefore, the majority of sources in a literature review are secondary sources that present research findings, analysis, and the evaluation of other researcher's works.
Reviewing secondary source material can be of value in improving your overall research paper because secondary sources facilitate the communication of what is known, the level of uncertainty in what is known, and what further information is needed from research. It is important to note, however, that secondary sources are not the subject of your analysis. Instead, they represent various opinions, interpretations, and arguments about the research problem you are investigating--opinions, interpretations, and arguments with which you may either agree or disagree with as part of your own analysis of the literature.
Examples of secondary sources you could review as part of your overall study include:
* Bibliographies (also considered tertiary);
* Biographical works;
* Books, other than fiction and autobiography;
* Commentaries, criticisms;
* Dictionaries, Encyclopedias (also considered tertiary);
* Journal articles (depending on the disciple can be primary);
* Magazine and newspaper articles (this distinction varies by discipline);
* Textbooks (also considered tertiary);
* Web site (also considered primary).