You can think of peer review as a "stamp of approval" from academic experts. When an article is published in a peer-reviewed journal, you can be certain that experts in the relevant field have read it and, independent of their own particular opinions, verified it meets a high standard of scholarship.
Scholars rely on peer review to ensure that the scholarship they exchange with each other is always based in good research and the established standards of their discipline.
The peer review system is similar to quality control systems that you see in everyday life. Just as you might be reassured to see a Health Department certificate in the window of a restaurant, the peer-review system provides an efficient standard of trustworthiness in academic scholarship.
See the short video below for a step-by-step explanation of how peer review works.
A primary source is an original object or document -- the raw material or first-hand information, source material that is closest to what is being studied. Scientific and other peer reviewed journals are excellent sources for primary research.
Primary sources vary by discipline and can include historical and legal documents, eye witness accounts, results of an experiment, statistical data, pieces of creative writing, and art objects.
In the sciences, the results of an experiment or study are typically found in scholarly articles or papers delivered at conferences, so those articles and papers that present the original results are considered primary sources.
A secondary source is something written about a primary source. Secondary sources include comments on, interpretations of, or discussions about the original material. You can think of secondary sources as second-hand information. If I tell you something, I am the primary source. If you tell someone else what I told you, you are the secondard source.
Secondary source materials can be articles in newspapers or popular magazines, book or movie reviews, or articles found in scholarly journals that evaluate or criticize someone else's original research.
Tertiary sources have the most fluid definition of the three levels of analysis. Generally speaking, tertiary resources analyze and synthesize information about a given topic. In other words, tertiary sources are information about information. They summarize the research on a particular topic in a user-friendly form or list primary and secondary sources.