Although written specifically for early career researchers, the Sense About Science guide “Peer review: the nuts and bolts” (pdf here) also gives an in-depth walk-through of peer review for journalists. Seeing what advice is given to early career researchers about the process can also provide reporters with insights about the goals and processes of peer review.
With a primer on the peer review process, including a short guide overview to peer review, this page also provides a number of other resources from Sense About Science to help journalists wrap their heads around peer review and the issues associated with the process and how it relates to open source publishing.
This Nature profile of Jeffrey Beall, an academic librarian and researcher at the University of Colorado in Denver, provides a good introduction to predatory journals and Beall’s list, one resource — albeit not unbiased — of potentially predatory journals.