Fast peer review process
Predatory journals often claim to offer a fast peer review, but in fact no review is made. Articles published in these journals are usually not taken into account in bibliometric or other assessments of the researcher's CV. It is also rare that they are cited in serious journals.
Website that may look serious
Predatory journals often have websites that look serious. They often claim to have editorial boards, impact factors and that they are indexed in well known databases.
In reality, their editorial boards may consist of individuals who cannot be considered to be qualified or who have not agreed to participate. Impact factors may come from various websites that provide fake impact factors.
Predatory publishers often use aggressive email marketing where researchers receive offers to publish in their journals. In many cases, they also have a conference activity linked to the publisher.
There are also examples of serious journals kidnapped by dubious operators who set up a website similar to that of the true journal. In some cases serious publications have been sold to questionable publishers.