In an article in ACRL News, CUNY librarians Monica Berger and Jill Cirasella describe predatory publishers as those who:
"...exist for the sole purpose of profit, not the dissemination of high-quality research findings and furtherance of knowledge. These predators generate profits by charging author fees, also known as article processing charges (APCs), that far exceed the cost of running their low-quality, fly-by-night operations."
Predatory publishers began profilerating in the past few years with the increase in open access publishing, and we are now also seeing an increase in predatory conferences, some which choose a name nearly identical to an established, well-respected conference.
There are ways to identify predatory publishers. The may claim to be included in directories and indexes when they are not and include faculty on their editorial boards who have not agreed to serve. The content of the journal may vary from the title and stated scope. The website for the publisher may lack contact information and appear unprofessional in quality. Finally, the peer review process for these journals may be expedited or not exist at all.
For more information on identifying quality journals, check out the Think, Check, Submit! link above as well as the database Cabell's.