A primary source or study documents an investigation that collects original data with the objective of gaining new knowledge on a topic. You might see primary literature also referred to as "empirical" or "original" research.
The authors of an article reporting study results are also the principal investigators who conducted the research. The study design can be quantitative or qualitative data and analysis.
When reading through primary studies, you can encounter a dizzying variety of terms describing primary studies - clinical trial, cross-sectional study, case-control study - just to name a few. The important thing to remember is that these are all original research and are considered primary sources.
(Hint: Some types are stronger than others. Keep an eye out for randomized controlled studies (RCTs) - they are generally recognized as the best of original study designs.)
The document below provides more detailed explanation of these and other types of primary research.
Secondary literature discusses, reviews and analyzes primary data. It is not considered original research. These sources are most useful as a starting point by providing an overview of a topic that might be unfamiliar to you.
Examples of secondary sources include reviews, encyclopedias, textbooks and websites.
Although reviews are secondary in nature, not all reviews are created equal.
A literature review provides a synthesis of existing research on a particular topic. It will often classify and compare prior research studies, reviews of literature and theoretical articles.
A systematic review, on the other hand, aims to identify and synthesize scholarly research on a particular topic in an unbiased, reproducible way. It begins with a clearly formulated question and uses systematic methods to identify, select and critically appraise relevant research studies.
Keep in mind that many databases also contain reviews of books, book chapters and singular articles. These are not the [reviews] you are looking for... move along. 
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