Historians distinguish between primary vs. secondary sources.
"Secondary" is fairly straightforward to define: what historians write about past phenomena. The best known examples are books and journal articles. Other formats can include papers in edited volumes, chapters, dissertations, and more.
"Primary" sources however take many forms--correspondence, diaries, personal naratives, photographs, official public records, news reports, advertisements... Primary sources are usually contemporary with the past phenomena under study.
It is important to remember that primary sources may be reproduced and published much later. Even if you are looking at a copy, it is still a primary source.