A systematic review is a methodical and comprehensive literature synthesis focused on a well-formulated research question. Its aim is to identify and synthesize all of the scholarly research on a particular topic, including both published and unpublished studies. Systematic reviews are conducted in an unbiased, reproducible way to provide evidence for practice and policy-making and identify gaps in research. They may involve a meta-analysis.
Systematic reviews are much more time-intensive than traditional literature reviews. They usually require a multi-person research team. Before embarking on a systematic review, it is important to determine whether the body of literature warrants one and to clearly identify your reasons for conducting a systematic review.
Narrative reviews articles may be evidence-based, but they are NOT evidence (research). They usually lack systematic search protocols or explicit criteria for selecting and appraising evidence. Instead, they rely on experts to gather evidence and synthesize findings.
PLoS Medicine (OPEN ACCESS) Moher D, Liberati A, Tetzlaff J, Altman DG, The PRISMA Group (2009). Preferred Reporting Items for Systematic Reviews and Meta-Analyses: The PRISMA Statement. PLoS Med 6(7): e1000097. doi:10.1371/journal.pmed1000097