Literature reviews are summaries of the literature on a particular topic. Reviews are generally considered "research", especially systematic and integrative reviews, but are not experimental in nature. There are several kinds of reviews: plain literature reviews, systematic reviews, and integrative reviews are the most common. Chapter 5 of Introduction to Nursing Research: Incorporating Evidence-based Practice (Cannon & Boswell, 2011, 2nd ed. Sudbury, Mass: Jones & Bartlett Learning) covers the purpose and process of a literature review in the context of writing a research article, thesis, or dissertation. How to undertake a literature search: a step-by-step guide (Watson, 2020, BJN, 29(7): 431-435) is a good overall guide.
Types of literature reviews:
*A meta-analysis study is one where carefully selected data from previous studies is combined to bring more rigor to a statistical or other analysis. No additional experimental work is done (usually). A systematic review is necessary to be sure that the data from the selected studies is comparable and combinable.
"Grey Literature" is the non journal article literature that isn't usually included in library database searches. It can include things like agency reports, grant proposals and reports, whitepapers, theses/dissertations, etc. It is SOMETIMES appropriate to include grey literature in a review.