A "fake" or "shady" journal is a low-quality journal that does little or no quality control.
They are often called "predatory" because they prey on people who are under a lot of pressure to publish, charging high author fees for promises of quick publication. (Since they do little, if any, peer review, the time from submission of a paper to publication is often very quick in these journals).
They often engage in deceptive practices to make themselves appear legitimate, such as:
- Pretending to be affiliated with a reputable professional society
- Claiming an "impact factor" when they do not have one, or when they have an "impact factor" from some entity other than Thomson Reuters ISI
- Listing important academics on their editorial board, when these people never agreed to serve in this capacity.
Publishing in one of these journals can be very damaging to your academic career. At best, it shows that you don't know what journals are considered reputable in your field; at worst, it makes you look like you are trying to "get" publications without doing the work required to publish in a reputable, high-quality journal.
An academic librarian named Jeffrey Beall keeps a list of open-access journals and publishers that he considers to be "predatory." His lists are quite well-known - you may hear people refer to "Beall's List" when talking about predatory journals.
Between the growth of open-access and the Internet making it possible for literally anyone to start a "journal," these "journals" have been popping up at an alarming rate. The NY Times even ran a story about it recently.