In essence, a literature review is a comprehensive overview of prior research regarding a specific topic. The overview both shows the reader what is known about a topic, and what is not yet known, thereby setting up the rationale or need for a new investigation, which is what the actual study to which the literature review is attached seeks to do.
from Denney, A. S., & Tewksbury, R. (2013). How to write a literature review. Journal of Criminal Justice Education, 24(2), 218-234. doi:10.1080/10511253.2012.730617