Primary and secondary sources often differ based on the academic discipline.
Primary Sources are original materials that contain firsthand testimonies. Primary sources include: letters and diaries, artifacts, photographs, interviews and reports, datasets, law documents, and original research. For example, the Bible is a primary source, as are the writings of theologians such as St. Augustine, Anselm of Canterbury, and Martin Luther.
Secondary Sources are materials that engage with primary sources but have additional analysis and contextualize. They are one step removed from primary sources. Secondary sources include: documentaries, scholarly books and articles, criticisms and reviews, textbooks, histories, and commentaries. Most of the materials you will use for writing papers will be secondary sources.
Tertiary sources are even more removed from primary sources. While primary sources are within a specific context and secondary sources analyze said context, tertiary sources instead take away context and analysis and give a broader overview of information. Tertiary sources include: dictionaries, handbooks (like the Chicago Manual of Style), bibliographies, timelines, encyclopedias, and many textbooks. These are the least specific types of sources, so they make a good starting point if you're unsure of your paper's specific focus.