"Literature reviews, including research syntheses and meta-analyses, are critical evaluations of material that has already been published. In meta-analyses, authors use quantitative procedures to statistically combine the results of studies. By organizing, integrating, and evaluating previously published material, authors of literature reviews consider the progress of research toward clarifying a problem. In a sense, literature reviews are tutorials, in that authors: define and clarify the problem; summarize previous investigations to inform the reader of the state of research; identify relations, contradictions, gaps, and inconsistencies in the literature; and suggest the net step or steps in solving the problem. The components of literature reviews can be arranged in various ways (e.g., by grouping research based on similarity in the concepts or theories of interest, methodological similarities among the studies reviewed, or the historical development of the field)."
From Publication Manual of the American Pyschological Association, 6th ed., section 1.02.