Definitions
Eureka Server
The discovery server. It contains a registry of services and a REST
api that can be used to register a service, deregister a service, and
discover the location of other services.
Eureka Service
Any application that can be found in the Eureka Server’s registry and is
discoverable by others. A service has a logical identifier sometimes
called a VIP, sometimes called a “service id”, that can refer to one or
more instances of the same application.
Eureka Instance
Any application that registers itself with the Eureka Server to be
discovered by others
Eureka Client
Any application that can discover services
Questions
How can an application be both a Eureka Instance and a Eureka Client?
Applications often need to make themselves available for use by others (so they are an instance) while at the same time they need to discover other services (so they are a client).
Does a Eureka Client have to be a Eureka Instance?
No. Sometimes an application has nothing to offer and is only a caller of other services. Via configuration (eureka.client.register-with-eureka=false), you can tell it not to register itself as an instance. It is therefore only a Eureka Client as it only discovers other services.