When we talk about “the literature” the body of research, scholarly articles, books and other sources (e.g. dissertations, conference proceedings) relevant to a particular issue, area of research, or theory.
The purpose of a literature review is to analyze critically this segment of a published body of knowledge through summary, classification, and comparison of prior research studies, reviews of literature, and theoretical articles.
Don't ignore quality paper resources in favor of easily accessible online materials. Many humanities and social science publications aren't available online, or the ULS may not have an online subscription to the publication you are seeking.
NCSU Libraries http://www.lib.ncsu.edu/tutorials/litreview/