As human beings, we are so used to interacting with each other that any world without humans would feel alien to us, including digital ones. In this article, we present a survey on the role of digital human-like characters in virtual worlds, both as counterparts of real human users and as embodied agents driven by artificial intelligence. The main issues related to 3-D graphics, physics, animation, and behavioral modeling are introduced, suggesting wherever available different alternatives and related development pipelines. A sizeable list of examples illustrating the use of virtual humans in different application sectors is then presented, focusing in particular on four domains: environmental design, training, cultural heritage, and healthcare.