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. What does that mean in the sciences? 

Secondary sources report on or interpret primary sources. What might that include for biology? 

Tertiary sources synthesize and present overviews of primary and secondary sources. What types of materials does this include? 

Scholarly sources present sophisticated, researched arguments and are written by experts.

Popular sources aim to inform or entertain and are intended for a general, non-specialized audience.