Tertiary sources - often called background or reference sources - are based completely on secondary sources and the research of others, rather than on primary sources or the author's original research. They pull together existing information on a particular issue or event, usually in an easy to read format, with the goal of synthesizing large amounts of information.
They are often excellent starting points for research projects because they provide useful information that is easy to access and understand. A good reference or tertiary source can provide:
Note that these types of sources are not peer-reviewed. They are intended to provide you with enough of an understanding of and vocabulary for your topic to allow you to successfully find more in-depth, scholarly information (like peer-reviewed books and journal articles).