This discussion covers only scientific, medical and technical literature. It is not meant to provide guidance in the arts, humanities and social sciences, where the meanings of 'primary literature' and 'secondary literature' are quite different.
In the sciences, primary literature reports the results of original research, research that finds something that no one knew before. It may be work done in a laboratory or observation of natural phenomena. Sometimes it gives the results of redoing work that someone else has already done just to see if it is correct. As one student said recently, "It's like a lab report."
Scientific primary literature usually has a title that summarizes the findings. Primary articles normally have the same or similar sections:
Secondary scientific literature describes or discusses primary literature. This can take the form of a literature review or meta-review, where the authors have read many primary articles to describe the current state of knowledge of a scientific topic. A news story about a new scientific finding is also a secondary article.
Some journals publish both primary and secondary articles. The two most important English-language science journals, Nature and Science, both do this. So you will need to carefully read the article to see what kind of article it is.
One excellent source of secondary articles only is the Annual Reviews series, many of which we have in full text. These annual publications cover one field, like biochemistry or genetics. Each volume has from 15 to 30 articles. Each article revised what we know about one topic. The bibliographies can run to a hundred to two hundred or more articles.