Information comes from many sources including:
These information sources differ in utility, content, intended audience, and authority -- among other things. However, there are only a couple of distinctions that are important to understand about information sources in science: scholarly vs. popular and primary vs. secondary.
Depending on the policies of a particular scholarly journal, before an article is accepted for publication in that journal, it must undergo peer review. The editor of the journal will identify a few other scholars (peers) who have expertise on the subject that the article is about and send a copy of the article to those individuals. The peers will review the article for quality and rigor and give the editor their comments and recommendations. Depending on what the peers say, the editor may reject the article (if the peers found it to be substandard), or ask the author to revise and improve the article based on recommendations the peers provided. If the article needs revision then the author will make the necessary alterations and then resubmit the article. The article will usually be accepted for publication if the author makes all the required revisions.
Review articles are a particular type of scholarly article that provide a systematic overview and analysis of the primary literature in a particular field. Review articles sum up the current state of research including the major advances and discoveries, ongoing debates, and where there are gaps in our knowledge that require more research. Because review articles discuss and cite the research articles that are advancing the field, you can quickly get an idea of who the major researchers are in that field.
CHARACTERISTICS OF SCHOLARLY SOURCES
In general, you can feel very confident about the quality and accuracy of information that you find in scholarly sources. However, scholarly sources can be challenging to use because the vocabulary and concepts are technical and specialized.
CHARACTERISTICS OF POPULAR SOURCES
It is not as easy to judge the quality and accuracy of information you find in popular sources (compared to scholarly sources). Your best bet is to stick with reputable, mainstream publications. Watch out for information that has been hyped or sensationalized.
In science, primary sources are those documents that report on new scientific findings. When researchers discover something new, they usually write up their findings and publish them in the form of an article in a scholarly journal. The objective of such an article is to share the new findings with others and advance that field of research. Scientific articles that report on new findings contain a section on the materials and methods used in the research project so that others can replicate and/or verify the results. Most journals that publish articles like this have policies in place to send the article out to be peer-reviewed before it is formally accepted for publication. The way peer-review works is that the journal editor will identify a few experts (peers) in the field, send them the article, and ask them to review the article to ensure the research is rigorous.
Primary sources are not always journal articles. New scientific findings might also be presented in conference papers or posters, technical reports, or dissertations. These types of sources are often called gray literature because they fall outside the formal scholarly publication process. However, they may still describe original research (which makes them primary sources). Quality standards for gray literature may be less stringent (or non-existent) so you should exercise more caution when using these materials.
Secondary sources provide a simplification, summary, or analysis of information that was originally derived from primary sources. Most scholarly books do not report on new scientific findings, but instead provide context, analysis, and theory -- so they are secondary sources. Some other examples of secondary sources are student textbooks, encyclopedia entries, and articles in popular science books or magazines.
A very important secondary source to know about is the review article. (See box to the left for details.)