A literature review is a thorough overview and critical analysis of the research already conducted and published on a specific topic. A literature review can be a standalone document written in order to explore the vast array of opinions, data, and voices concerning your topic of research. When writing for your thesis or dissertation, a literature review will be just one part of a much larger document, written to situate your research in the larger context of the topic itself. Diana Ridley in her text The Literature Review: A Step by Step Guide for Students explains that,

your research is a small piece in a complicated puzzle jigsaw puzzle; it does not stand alone. It is dependent on what others have done before and will contribute to an ongoing story or debate. Your reader therefore needs to know about the whole jigsaw puzzle and not simply the shade or shape for your particular piece. In a literature review, you are contextualizing; you are describing the bigger picture that provides the background and creates the space or gap for your research (2008; pg. 5).

Therefore, the literature review should be more than just a summary of the resources and include, for example, interpretations, comparisons, evaluation, and clarification, of the research already performed on the topic. ​

A literature review as both product and process

Ridley also explains that the term literature review describes both a final product and a process itself. 

As product

As a noun, the literature review is part of the final draft of your standalone document or part of your larger thesis or dissertation. It is the piece of writing that allows the reader to know that you have engaged in deep research of your topic. It also allows you to build a dialogued with the research performed before yours, by offering an analysis of those works in relation to your own. This piece of writing will allow you to identify ideas, theories, terminology, or the timeline of thought about the topic as it has developed over time (Ridley, 2008, pg. 2). This piece of writing is where you are headed as you embark on the literature review process. 

As process

As a verb, the literature review is a continuous activity which begins as soon as you have selected a topic and begun your research (Ridley, 2008, pg. 3). While reading and collecting data, you will keep track of your findings in some sort of a research log or citation management system of your choice. Every author or idea you collect is an opportunity to build a connection in the larger research conducted on the topic and might hold a place in your literature review.