Preprinting has gained considerable momentum, and in some fields it has turned into a well-established way to share new scientific findings. The possibility to organise quality control and peer review for preprints is also increasingly highlighted, leading to the development of preprint review services. We report a descriptive study of preprint review services with the aim of developing a systematic understanding of the main characteristics of these services, evaluating how they manage preprint review, and positioning them in the broader scholarly communication landscape. Our study shows that preprint review services have the potential to turn peer review into a more transparent and rewarding experience and to improve publishing and peer review workflows. We are witnessing the growth of a mixed system in which preprint servers, preprint review services and journals operate mostly in complementary ways. In the longer term, however, preprint review services may disrupt the scholarly communication landscape in a more radical way.