Peer review is the process an article or book goes through before being published. Other experts and researchers will evaluate a work to determine its validity and accuracy before the it is published, either in a scholarly journal (an article) or through a college/university press (a book). Peer-reviewed sources may also be called refereed or juried. The intended audience is usually other professionals, researchers, experts, or students.
Ulrich's Web is a database that can give you information about a journal title, including whether the title is peer-reviewed. (See the FAQ on Ulrich's.) You can also look at a journal's website. Some of the Library's subscription databases may also tell you whether a title (not necessarily a particular article) is peer-reviewed, in the item record.