Popular sources are usually written by journalists and other non-experts for a lay audience. Time Magazine and The New York Times may report on scientific topics, but you'll eventually want to get to the actual scholarly source that these articles are based on.
Scholarly sources are often written by academics, professional researchers and other experts, for a specialized audience. These are the kinds of articles you'll want to focus on for this course -- and you'll want to find primary or secondary sources.
Primary literature papers are reports on the results from original research by the people that performed the experiments. These papers contain data from a small set of experiments that tells a defined story, but is usually part of a larger problem being addressed by the lab.
Secondary literature, such as review articles, is different in that those papers summarize the findings of many research reports from many different researchers. Reviews are useful to get a broad picture of a research topic. However, the writers of reviews sometimes have an agenda they are trying to push and might not include results that don’t support their theory. Textbooks and other specialized, topical science books can also be considered secondary sources.
Tertiary literature can include fact books and reference books such as encyclopedias (some consider these secondary). The information in these is derived from primary and secondary literature.