Most publishers own the rights to the articles in their journals. Anyone who wants to read the articles must pay to access them. Anyone who wants to use the articles in any way must obtain permission from the publisher and is often required to pay an additional fee.
Although many researchers can access the journals they need via their institution and think that access is free, in reality it is not. The institution has often been involved in lengthy negotiations around the price of their site license and reuse of this content is limited.
Open Access journals today vary widely across a spectrum based on the core components of reader rights, reuse rights, copyrights and more. The highest possible level of Open Access means that articles are immediately and freely available to anyone, anywhere, to be downloaded, printed, distributed, read, reused and remixed (including commercially) without restriction, as long as the author and the original source are properly attributed according to the Creative Commons Attribution License (CC BY).
Predatory Open Access Journals "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.
Charging a fee is not itself a marker of a predatory publisher: many reputable OA journals use APCs to cover costs, especially in fields where research is often funded by grants. (Many subscription-based journals also charge authors fees, sometimes per page or illustration.) However, predatory journals are primarily fee-collecting operations—they exist for that purpose and only incidentally publish articles, generally without rigorous peer review, despite claims to the contrary."
DOAJ is considered a Open Access Journal whitelist, listing journals it has vetted and found to be of good quality, while Beall's List lists OA journals it deems "Potential, possible, or probable predatory scholarly open-access publishers."