Primary sources are documents and objects which were created at the time under study or they are original research (experiments, statistical data, scientific studies, surveys). They are different from secondary sources which are accounts, interpretations, reviews or criticism of primary sources or historical events.
-- Adapted from the Library of Congress
Primary Source Examples in the Humanities
Primary Source Examples in the Sciences
Secondary sources may include biographies, textbooks, histories, literature review articles, and criticism. They analyze, review, or interpret primary sources.
When you are doing a research project, you want to include a mix of primary and secondary sources. Your instructor wants your own original thoughts on the topic. Doing your own analysis or synthesis of primary sources will help you accomplish this and avoid the temptation to overly rely on other people's interpretation of primary sources (which can happen if you only use secondary sources).
At this same time, you do not want to only use primary sources. Using scholarly secondary sources helps you to understand the ongoing scholarly conversation around your topic and to make sure that your research is engaging with this ongoing conversation.