An article in a Peer Review journal (also called a "refereed journal" is one that has been looked at carefully before publication by experts in a scholarly field (the article author's "peers" in the profession) to determine if the article meets standards of research in a given field and allow it to be published. Peer reviewed or refereed is a mark of quality.
Articles that have been peer reviewed are claiming to be presenting original research, although it is no guarantee that the article is only presenting valid claims.
Academic Search Complete is a large database with mainly scholarly sources. Google Scholar is even bigger. Although there is a filter for "Peer Reviewed" in Academic Search Complete, you might be better off starting your searches in databases that are limited almost entirely to peer reviewed journals. For example, the database JSTOR and Social Science Citation Index are good for this.
There are other things in peer reviewed journals that are not considered research, such as editorials, letters to the editor, book reviews and announcements. These other things may turn up in a database search even when you use a limit filter such as "Peer reviewed." If you have any doubt about something, ask your professor.
Publications that don't use peer review use the judgment of the editor and fact-checkers to decide on the quality of the article. If they do mention original research, these publications are usually only reporting on research done by researchers rather than the researcher presenting the research for the first time -- even if it is written by someone with academic credentials. Foreign Affairs, The Economist. and CQ Weekly (for federal and Congressional news) are examples of these kinds of publications.
Web of Science is an excellent tool for tracking down who has used a paper in their own research ("cited" or used as a reference.) Google Scholar has a similar "cited by" link that leads to a much wider (less selective) set of material -- including books. It is interesting to compare results. Annual Reviews articles are literature reviews on different topics that put research in context and organize it into themes. Finding an article on your topic here can be a great way to put your own project in context.
Why cited articles are important: Political Science research is first presented by researchers in academic journals and books. (Sometimes, research with a general interest will get reported in magazines and newspapers and this can be a big help to understanding. See the tab Other Resource Types and ideas.) The more an article is cited is a measure of its importance to other scholars.
But most research grows out of the eye of public awareness. As research interest in a topic grows, there begins to be a group of scholars who begin to connect with each other in a pattern you can see. The pattern emerges in who they cite or refer to in their work.
This citation pattern is called an invisible college and if you am going to write about a topic, you should get some awareness of who these scholars and their works are. Some scholars rise to prominence and have research that most other scholars refer to as the most accepted research. These are the big names you should know.
So, look in two different directions-- at the older foundation works, the most accepted research, and the very new research to see who is being cited most now.
