What are primary sources?

Primary sources are sources written (or made) by an original author. In exact sciences, these are lab logs, articles in which you present your research, the data you gathered during your research, etc. In social sciences these could be novels, paintings, archaeological objects, and so on.

What are secondary sources?

Secondary sources refer to one or several primary sources and elaborate on them. In exact sciences these are review articles. In social sciences these could be works about literary works, e.g. an analysis of the novels of Toni Morrison.

What are tertiary sources?

Tertiary sources give an overview of what can be found in primary and secondary sources. They are sources such as encyclopaedia, dictionaries, guides, bibliographies, etc. Within academic research, it is better to avoid referring to a tertiary source.

When do you use which sources?

It is useful to define which types of sources you need at the start of your search. Do you still need to explore the research field? Then secondary sources are interesting; review articles of articles and of books can point you in the right direction. This way you do not have to read and digest entire studies to know if they might be relevant to your research.

Once you have an overview of your field, you can delve a bit deeper. Start reading primary sources that you found in the review articles. Note that the exact sciences focus more on articles than entire monographs (books on a specific subject authored by one or more researchers). You should cite the primary sources when you write your research paper.

And lastly, you can keep an eye on the latest developments by reading the new articles. Articles take less time to get published than an entire book, especially in social sciences, where monographs are common.