In addition to thinking about the type of source you might use (e.g., a book or a scholarly journal article), information is also classified as being a primary, secondary, or tertiary source.
When considering whether a source is a primary, secondary, or tertiary source, look at how close an author or creator of information is to the event portrayed.
Primary sources are original materials. They are from the time period when the event occurred and provide first-hand testimony or evidence. They present original thinking, report on discoveries, or share new information.
Examples include: diaries, letters, speeches, works of literature, scientific journal reporting original research, original documents, like a birth certificate or a patent, raw data or statistics, etc.
Secondary sources interpret, analyze or summarize. They have been created after the fact with the benefit of hindsight. They comment on or discuss the evidence provided by the original work.
Examples include: biographical works, commentaries, magazine articles, review articles, scholarly books, popular non-fiction, etc.
Tertiary sources summarize and condense. They aim at organizing known information to put it into a convenient, easy-to-read form.
Examples include: Bibliographies, dictionaries and encyclopedias, directories, etc.