A systematic review is a comprehensive search of all of the studies, papers, research, and literature revolving around a specific question. It attempts to eliminate any bias, and includes unpublished studies ("grey literature"). The reviewers filter out poorly done studies and make recommendations based on a synthesis of the high quality studies.
The ultimate goal of a systematic review is to change and improve clinical practice.
A systematic review is more detailed than a literature review or a meta-analysis. For more information on the different types of reviews, read Grant and Booth's article "A Typology of Reviews."
Note that if you want to do a meta-analysis, a systematic review must be done first.