In the sciences, as well as some social sciences, primary sources are original articles or other texts describing a researcher's new experimental data, observations, results, and/or theories. These primary articles usually contain an introduction, methods, and results section. As a note, some review articles (which are not primary sources) may also contain these sections, so the reader should always review the article's content to determine what kind of article it is. An article can appear in a scholarly peer-reviewed journal and not be a primary source (i.e. editorials, review articles, book reviews).
A secondary source is an interpretation or analysis of the document, object, or event, at a later date. Secondary sources comment on or review one or more primary sources. Secondary sources are at least one step removed from the primary source.
Examples of secondary sources in the humanities and some social sciences include: scholarly or popular books, journal articles analyzing primary sources, documentary movies, etc. Examples of secondary sources in the sciences and some social sciences include newspaper articles summarizing an experiment and review articles summarizing many primary articles on the same subject.
You may be wondering what we mean when we refer to scholarly versus popular.
Learn more: What is Scholarly?
While we're at it, how about a few more key terms...