Predatory journals are open-access, web-based journals that charge the author a fee (sometimes $1000's) to "publish" an article but fail to provide traditional editorial & publishing services (i.e. peer review, ISSNs, etc.). Although the U.S. government is beginning to crack down on the practice, the desperate need to "publish or perish" faced by scientific researchers feeds into this exploitation
Beall's List provided a running A-Z list of "potential, possible, or probable predatory scholarly open-access publishers." The list went defunct in 2017 but an archived list is linked above.
Some question the criteria through which Beall and his adherents make their selections of who is and is not considered predatory. Beall has been named in law suits.