By "secondary source" researchers mean articles or books that discuss a person or event: while primary sources may have been created at the time of an event by a participant, in a secondary source a historian will gather the primary sources, create a coherent narrative, and draw conclusions from the evidence.
For your research, look for scholarly, reliable sources: has the article or book been written by an expert in the field? is it a recent work, or has it been overtaken by more recent historical finds?
Scholarly sources almost always include footnotes, endnotes, reference lists or bibliographies. This is done to give proper credit to the original sources of information and to provide other scholars with a "paper trail" to follow to the original sources if desired. Non-scholarly sources occasionally include these features as well, but they are typically not as extensive.
The intended audience for scholarly sources is other experts and serious students in the same field rather than general readers or less advanced students. The language may be technical or otherwise challenging to understand without some background knowledge. But some scholarly sources are written in a more engaging style and are accessible to a wider range of readers.
Scholarly publications are written primarily to advance knowledge. Though authors may support one point of view over another, their arguments are based on evidence rather than personal opinion. By contrast, popular publications are often written to entertain, air personal views, and sell the publication.
Appearance can also provide many clues. Scholarly journals usually have a plain rather than decorative cover; ordinary rather than glossy paper; charts, graphs, and occasional illustrations rather than an abundance of graphics; and little or no advertising. Articles tend to be long, and the journal is published quarterly or semi-annually rather than weekly or monthly.
These distinctions apply to secondary sources for the study of history. Primary sources are selected for their relevance as original evidence that addresses historical questions rather than whether they are scholarly or not.
