A primary source is a document that was created at the time of an event (newspaper article, letter, diary, pamplet, art) or was produced later by a person who participated in that event (memoir or oral history). These are the closest documents to an event.
Secondary sources are created using primary sources as evidence to support theories or interpretations of events. These come in a wide variety of formats such as books, essays, commentaries, journal articles, or newspaper articles. One way to distinguish a secondary source is the presence of a bibliography and footnotes.