Students assigned research papers are often confused if told by an instructor to use primary sources.
Here's a quick take on the differences between the three source types.
Autobiography of Ben Franklin courtesy of the Library of Congress
Use Google Scholar to find academic-quality information (articles, papers, reports) on the Web.
A QUICK LOOK: Clarification by Example / Charles Darwin
A PRIMARY SOURCE is an ORIGINAL SOURCE or STUDY. It's fresh in its originality and is often from a past moment or era in history. It has not been filtered through analysis by others.
EXAMPLE: Charles Darwin's notes, letters and rough drafts of his arguments in behalf of natural selection and his theory of evolution, all of which led to his well-known published works, such as The Origin of Species (1859). These items contain the author's unprecedented observations of, and thoughts about, the natural world and how its inhabitants evolved physiologically.
NOTE: Many historical primary sources are available online. For example, Darwin's primary works can be found on the Web in many places. Here's one excellent source: The Complete Works of Charles Darwin Online (website).
A SECONDARY SOURCE analyzes, or "filters," one or more primary sources, often, but certainly not always, years after these primary sources first appeared.
EXAMPLE: A 2005 book analyzing and commenting on Darwin's observations, thoughts and theories is entitled Darwin: Discovering the Tree of Life (print), written by Niles Eldredge. It takes a detailed look at Darwin's primary works, making it a SECONDARY source.
A TERTIARY SOURCE compiles information from primary and secondary sources and presents an article of limited length that appears among a collection of articles on numerous other related subjects.
EXAMPLE: The Biographical Encyclopedia of Scientists (print). It contains a two-page article summarizing important facts about, and accomplishments of, Charles Darwin. This is a TERTIARY source.
Another TERTIARY source that provides brief coverage of Darwin is The Cambridge Dictionary of Scientists (link takes you to an explanation of the book).
NOTE: The examples above are intended to provide a basic understanding of the three types of sources. BUT it's important to remember that what constitutes a primary, secondary, or tertiary source can vary from one academic discipline to another, as is illustrated behind the Discipline Grid tab above.