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The following definitions refer to published, available information. The information is put in a (reasonably) permanent form.
Primary Sources are firsthand reports by a person(s) who was present.
More specifically,
Primary Sources are the original sources that first report
as reported by the person(s)
In research, primary sources are often research articles in scholarly journals. However, primary sources may include newspaper articles, research reports, trade journal articles, conference proceedings, dissertations, Web sites, novels, poems, plays, speeches, interviews, letters, case studies, test data, findings from surveys, archaeological drawings, experiments, films, drawings, designs, paintings, music, sculptures, etc. IF the source is the *original* source of information as told by the person who did it or developed it.
Primary sources can vary by discipline. But a primary source is always a firsthand source by someone who was there at that place and time.
EXAMPLES
In history, a primary source would be a paper, video, or recording of an event of interest. It would be a published paper or recorded audio by someone who was there and witnessed the event. It would be a video or photograph of the event. So you would be getting a firsthand account of the event.
In science and medicine, a primary source is an original research study as written/told by the original researchers. So a primary source is a report of a research study by the researchers who did the research. Researchers most often publish their research in scholarly peer-reviewed journal articles of their discipline. But some original research studies may be shared via other sources.
Secondary Sources are secondhand reports by person(s) who did not learn about the thing firsthand. They are sources that analyze, describe, comment on, and synthesize the primary or original source and/or other secondhand sources. So they are sources that are about someone's else's original work, observations, writings, etc. These include review articles, newspaper articles, reference books such as encyclopedias and dictionaries, and textbooks.
This definition is a little vague because there is some disagreement among experts regarding the distinction between secondary sources and tertiary sources. But because you may encounter the phrase "tertiary sources," it helps to know what it means.
Tertiary sources are resources written about the secondary literature. In other words, literature which explains, teaches you how to use, and leads you through the vast array of primary and secondary scientific literature. Examples may include textbooks, monographs, bibliographies, encyclopedias and reference books of all kinds which provide a summary of accepted knowledge about a topic or subject area in broad outline. Librarians disagree on the precision of this definition and some do not use the term at all, preferring categories of only primary and secondary sources. But when they do use "tertiary source," this is what they mean.
What is "grey literature?" (Or "gray.") Grey literature is information that has not been formally published and, therefore, is unavailable or hard to find. A good analogy is “as a shadow which has not yet acquired substance”. Also called “fugitive literature,” it usually refers to knowledge that is out in the world but has not yet been formally written down and distributed. For example, researchers may discuss a research study with colleagues and references may turn up in that person's paper but the original study itself is not available in a permanent form. Sometimes the information in grey literature never becomes available to you! For example, if a pharmaceutical company is researching a new drug and doesn’t distribute the information outside of its own employees because it doesn’t want to help the competition. Or a researcher never bothers to publish their research findings for some reason.
Grey literature publications include theses, conference proceedings, technical specifications and standards, non-commercial translations, bibliographies, technical and commercial documentation, government documents, and reports (pre-prints, preliminary progress and advanced reports, technical reports, statistical reports, memoranda, state-of-the art reports, market research reports, etc.). (Alberani, 1990)
The Grey Literature Report of the New York Academy of Medicine notified subscribers of grey literature publications in health services research and selected public health topics from 1999-2016. You can search their Grey Literature Collection as if you were searching a library catalog. Records of results of interest would have to be retrieved like any other book. It also listed organizations that publish grey literature.
This is a nice video by a librarian about scientific primary sources. However, she recommends databases that are general databases in order to find primary sources in the sciences. Those databases are decent databases on a variety of topics, but they do not specialize in the science disciplines. You may wish to use the ISU Science Databases to find science publications.