The APA definition of a literature review (from http://www.apa.org/databases/training/method-values.html):
Survey of previously published literature on a particular topic to define and clarify a particular problem; summarize previous investigations; and to identify relations, contradictions, gaps, and inconsistencies in the literature, and suggest the next step in solving the problem.
Literature Reviews should:
Adelphi Library's tutorial, Conducting a Literature Review in Education and the Behavioral Sciences covers how to gather sources from library databases for your literature review.
The University of Toronto also provides "A Few Tips on Conducting a Literature Review" that offers some good advice and questions to ask when conducting a literature review.
Purdue University's Online Writing Lab (OWL) has several resources that discuss literature reviews:
https://owl.english.purdue.edu/owl/resource/994/04/ (for grad students, but is still offers some good tips and advice for anyone writing a literature review)