"Megajournals" are relatively new in the scholarly publishing world. These open access journals publish articles in a wide range of subjects. PLOS One, which publishes thousands of articles per month, was the trailblazer of this development. Like traditional journals, articles submitted to these journals go through rigorous peer review.
The primary difference in the review process is the objective editorial criteria used for acceptance or rejection of a potential article. Reviewers ensure that the research is scientifically sound and ethical, but do not weigh the importance or relevance of the work. This allows a much higher acceptance rate. It also allows the acceptance of some types of articles that are normally rejected, most notably negative results, replication studies and protocols.
For more information on megajournals, read Open Access MegaJournals - Have They Changed Everything? by Peter Binfield, the co-founder of PeerJ and previously the publisher of PLOS One.