The literature review is a written explanation by you, the author, of the research already done on the topic, question or issue at hand.
What do we know (or not know) about this issue/topic/question?
The review should enumerate, describe, summarize, objectively evaluate and clarify this previous research.
It should give a theoretical base for the research and help you (the author) determine the nature of your research.
It helps to think in terms of the manageable library of data that you are carefully building, as opposed to the vast ocean of data in which it will probably feel like you’re drowning.
There are some simple rules that can be applied: The quality of your review will sink or swim on the efficacy with which you select your search keywords.
It’s never a good idea to build the list quickly or on your own.
Look at other dissertations for their keywords, and reach out to colleagues, departmental faculty, your supervisor, and several librarians (even if they work at other institutions).
They become paranoid at the prospect of missing the latest paper or opinion piece that an examiner will spring on them at their oral defenses and destroy their academic careers in one fell swoop.
That’s a valid, albeit a little emotional, concern and can be easily managed through a well-planned selection of RSS feeds and journal alerts based on appropriate keywords.