If this is the first encounter with the tool, it will throw a lot of new terms at administrators, yet to be understood. While it follows VMware's methodology and naming conventions, there are a couple of things which are not used by any other tool in the VMware ecosystem.
Besides the portal itself, vRA requires some helper services to actually get things done in the underlying environment. During the setup, those are configured and aligned to work together with vRA to be able to automate the underlying infrastructure.
DEM is sometimes also referred to as the manager service. Basically, this component is connecting vRA to possible deployment targets for VMs. This can be vCenter (as suggested during the wizard-driven installation for vRA) but it can also be other hypervisor targets such as Hyper-V or KVM. Besides that, vRA will also be able to connect to external clouds such as Amazon Web Services (AWS), vCloud Air (VMware), and Microsoft Azure, as well as...