Which types of publications are "scholarly"?
Within medical and health sciences disciplines, "the literature" usually means only resources published in "academic" or "scholarly" journals. This is partially because of tradition, and partially because scholarly journals are where most research is reported, though people also present the results of their research in short reports or at professional conferences.
What makes a journal "academic" or "scholarly"?
Traditionally, academic institutions, or scholarly societies, were the only organizations who published the results of research. In the last several hundred years, this has changed, and now for-profit companies own and operate a lot of scholarly and academic journals, but they are still referred to as "scholarly" to signify their scientific rigor. This is opposed to trade journals, which publish about other things.
Types of publications:
Scholarly Journals publish scholarly articles, as well as commentary pieces and book reviews. However, when people say "scholarly article", they usual mean a research article, not commentaries and book reviews. Those are not usually considered appropriate for use in class assignments unless professional or social context can provide important context for discussion.
Scholarly (or academic) articles:
Original research articles: report, for the first time, the results of research undertaken by the authors writing the article. This is what makes them "primary" research. Case studies and clinical trials are two examples of original research.
Review articles: Also sometimes called "secondary" research, review articles report the results of research which gathered existing articles and summarized the research across those articles in order to make a conclusion about a particular research topic, process, or intervention. There are many types of review articles, and the types differ depending on the discipline. Here are just a few within the context of medical or health sciences: