Predatory, open-access publishers are those that unprofessionally exploit the author-pays model of open-access publishing (Gold OA) for their own profit. Typically, these publishers spam professional email lists, broadly soliciting article submissions for the clear purpose of gaining additional income. Operating essentially as vanity presses, these publishers typically have a low article acceptance threshold, with a false-front or non-existent peer review process. Unlike professional publishing operations, whether subscription-based or ethically-sound open access, these predatory publishers add little value to scholarship, pay little attention to digital preservation, and operate using fly-by-night, unsustainable business models.
This comes from a blog post by Jeffrey Beall, an academic librarian at the University of Colorado Denver. Find out more on his Metadata blog.