Searching the scientific literature can be challenging – thinking of the best keywords and concepts, finding the best resources, and accumulating a strong set of articles is a process that takes time and practice.
Literature is a term used frequently in academic and library circles that just means "the stuff that's been published on a topic."
There are three basic “levels” of literature:
Primary Literature: accounts of original research, written by the researchers who performed it. Primary literature must provide enough information to allow another researcher (with the right skills) to reproduce the experiment. Examples: journal articles, conference proceedings, dissertations, and patents.
Secondary Literature: This type of literature refers to the primary sources, in some cases synthesizing information from primary sources to draw new conclusions, or repackaging it in a new form. Generally, secondary literature provides some new information or criticism on the topics discussed. Examples: monographs (a fancy word for books), encyclopedias, some textbooks, and annual reviews.
Tertiary Literature: Tertiary literature is primarily used as a finding aid for primary and secondary literature, and does not usually provide any new information. Examples: directories, indexes, and bibliographies
In terms of importance for scientific research, the most important type is primary literature. Secondary and tertiary literature are there to assist you in locating and understanding the primary literature, but it is the first-hand accounts of actual research that let you know what has been done, and what researchers have discovered.