In academic research, it's important to be able to distinguish between different types of sources. These differences often are contextual, meaning that a single source might fit in different categories depending on how you are using it and in what academic discipline you are writing.
Primary sources are the raw materials of scholarship.
Secondary sources report on or interpret primary sources.
Tertiary sources synthesize and present overviews of primary and secondary sources.
Scholarly sources present sophisticated, researched arguments using both primary and secondary sources and are written by experts.
Popular sources aim to inform or entertain and are intended for a general, non-specialized audience. In academic writing, popular sources most often are analyzed as primary sources.