Review articles are secondary sources that summarize the current knowledge on a topic. They are useful when you don't know a lot about a topic or need a broad overview.
Here is just one example of a review article: http://libproxy.boisestate.edu/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=eih&AN=9606240326&site=ehost-live