A literature review is a critical evaluation of the research on a particular topic. It is a synthesis of what the existing research on your topic says- your perspective guides how you put that existing research together- the purpose is to identify a gap in in the research in order to identify areas for future research and progress within the topic.
Check out this short video for a more visual explanation of what a literature review is:
Tips, Help and Tutorials to guide your literature review process
These are links to various sources from around the web with information, strategies and tips for writing a literature review.
This page from Walden University gives examples of different types of literature review matrices. A matrix can be very helpful in taking notes and preparing sources for your literature review.
There are also two other short videos from the University of Maryland University College which give an introduction to both what a literature review is and isn't, and how to begin your writing process.