A literature review is important because it:
Different projects involve different kinds of literature reviews with different kinds and amounts of work. And, of course, the "end products" vary.
NCSU Libraries http://www.lib.ncsu.edu/tutorials/litreview/
Writing a literature review will take time to gather and analyze the research relevant to your topic, so it best to start early and give yourself enough time to gather and analyze your sources. The process of writing a literature review usually covers the following steps:
Keep in mind that a literature review defines and sets the stage for your later research. While you may take the same steps in researching your literature review, your literature review is not:
Not an annotated bibliography in which you summarize each article that you have reviewed. A lit review goes beyond basic summarizing to focus on the critical analysis of the reviewed works and their relationship to your research question.
Not a research paper where you select resources to support one side of an issue versus another. A lit review should explain and consider all sides of an argument in order to avoid bias, and areas of agreement and disagreement should be highlighted.