Review (in general), as defined by MeSH is:

"An article or book published after examination of published material on a subject. It may be comprehensive to various degrees and the time range of material scrutinized may be broad or narrow, but the reviews most often desired are reviews of the current literature. The textual material examined may be equally broad and can encompass, in medicine specifically, clinical material as well as experimental research or case reports. State-of-the-art reviews tend to address more current matters. A review of the literature must be differentiated from HISTORICAL ARTICLE on the same subject, but a review of historical literature is also within the scope of this publication type."

Source:  http://www.ncbi.nlm.nih.gov/mesh/68016454

What is a narrative review?

"Narrative reviews generally are comprehensive and cover a wide range of issues within a given topic, but they do not necessarily state or follow rules about the search for evidence.  Also typical narrative reviews do not reveal how the decisions were made about relevance of studies and the validity of the included studies."

Source:  Collins J, Fauser B. Balancing the strengths of systematic and narrative reviews. Human Reproduction Update [serial online]. March 2005;11(2):103-104. Available from: MEDLINE, Ipswich, MA. Accessed November 10, 2011.

What is a systematic review?

"A review of a clearly formulated question that uses systematic and explicit methods to identify, select, and critically appraise relevant research, and to collect and analyse data from the studies that are included in the review. Statistical methods (meta-analysis) may or may not be used to analyse and summarise the results of the included studies"

Source: http://www.cochrane.org/glossary/5#term164

Example systematic review
(Figure 1 a nice process diagram)

What is a case report?

A case report is a descriptive study of a single individual (case report) or [individual reports from a ] small group (case series) in which the possibility of an association between an observed effect and a specific environmental exposure is based on detailed clinical evaluation and histories of the individual(s).                             

Alternately, NLM provides this defintion: Clinical presentations that may be followed by evaluative studies that eventually lead to a diagnosis.

Source: http://www.ncbi.nlm.nih.gov/mesh/68002363

In a Medline search, Case Report can be entered as subject term. Alternately search results can be limited to those publication types identified as Case Reports.

What is a randomized control trial?

"Work consisting of a clinical trial that involves at least one test treatment and one control treatment, concurrent enrollment and follow-up of the test- and control-treated groups, and in which the treatments to be administered are selected by a random process, such as the use of a random-numbers table."

Source: http://www.ncbi.nlm.nih.gov/mesh/68016449

What is a placebo? [placebo might be used as the "control" treatment]

"Any dummy medication or treatment. Although placebos originally were medicinal preparations having no specific pharmacological activity against a targeted condition, the concept has been extended to include treatments or procedures, especially those administered to control groups in clinical trials in order to provide baseline measurements for the experimental protocol." [MeSH term]

Source: http://www.ncbi.nlm.nih.gov/mesh/68010919

What is a case-control study?

"Studies which start with the identification of persons with a disease of interest and a control (comparison, referent) group without the disease. The relationship of an attribute to the disease is examined by comparing diseased and non-diseased persons with regard to the frequency or levels of the attribute in each group." [MeSH term]

Source: http://www.ncbi.nlm.nih.gov/mesh/68016022

What is a cohort study?

"Studies in which subsets of a defined population are identified. These groups may or may not be exposed to factors hypothesized to influence the probability of the occurrence of a particular disease or other outcome. Cohorts are defined populations which, as a whole, are followed in an attempt to determine distinguishing subgroup characteristics." [MeSH term]

Source: http://www.ncbi.nlm.nih.gov/mesh/68015331

Cohort and case-control studies are both observational studies. This figure may help distinguish the two: