A literature review is a collection of selected articles, books and other sources about a specific subject. The purpose is to summarize the existing research that has been done on the subject in order to put your research in context and to highlight what your research will add to the existing body of knowledge. Literature reviews are typically organized in some way (chronological, thematic, methodological).
Let's take a look at an example:
Grant, M. J., & Booth, A. (2009). A typology of reviews: An analysis of 14 review types and associated methodologies. Health Information & Libraries Journal, 26(2), 91-108. doi:10.1111/j.1471-1842.2009.00848.x
These guides provide an overview of the literature review: the purpose of a lit review, what to include in one and how to organize one.