Colloquially

• ServiceEntry - what Services exist and how are their network endpoints found?
• VirtualService - to which Service should traffic for a given hostname be sent?
• DestinationRule - how do I talk to that Service?

Services are the destination of Route Rules (the host which a VirtualService matches is an arbitrary string). Services by the Istio definition are morally "units of application behaviour". They are made of network endpoints: Pods, and external endpoints. Istio has a Service Registry of all the Services it knows about. In some configurations it will refuse to send traffic to Services that aren't in the Service Registry (in others it'll assume the Service exists and take some guesses about how to find its endpoints and how to talk to them).

In a Kubernetes environment, the Service Registry is populated from Kubernetes Service instances. Extra Service Registry entries (eg for internet endpoints, VMs) can be added with the ServiceEntry resource. Recall that, in some mesh configurations, such ServiceEntries are necessary for workloads in the mesh to talk to anything outside the cluster. ServiceEntry is a higher-level resource - doesn't necessarily specify all endpoints in a Service, instead it says where to discover them.

There is no (easy) way to view the contents of the Service Registry; the imported Kubernetes Services, DNS query results, etc aren't reflected as a read-only resource like Kubernetes' Endpoint.