The scientific literature is the collective body of knowledge that has been published since the beginning of modern scientific practice in the mid-17th century. The literature is truly vast, containing hundreds of millions of documents in all kinds of formats and languages, on every topic imaginable. The literature's growth is exponential - one study estimates it doubles in size every nine years - meaning that the vast majority of articles out there have appeared in just the last few decades.
The literature is both produced and consumed by scientists who use it to inform their own research, which is then in turn reported for others to use. This is called the Research-Publication Cycle, an endless loop of discovery and learning (moving counterclockwise in this diagram) and knowledge dissemination (moving clockwise).
Primary Literature: This is where NEW research is reported. It includes preprints (articles submitted but not yet peer-reviewed); rapid communications (often called letters or reports); peer-reviewed full articles in journals; papers presented at conference and symposia; patents; and academic theses (which describe a graduate student's long-term research project, some of which may already have appeared as journal articles).
Secondary Literature: No new results are reported, but rather a synthesis and assessment of recent research on a particular topic. This can be review articles in journals or review series, or as chapters in an edited book, or a full-length monograph.
Tertiary Literature: Some of the knowledge reported in the primary literature may eventually make its way into publications such as encyclopedias, handbooks, and the like. It also includes academic textbooks.