Eureka or Eureka Server is an application that holds the information about the client-service applications. 

Microservices have to register into the Eureka server, and the Eureka server knows all the client applications running on each port and IP address. 

Eureka Server is also alternatively known as the Netflix Service Discovery Server. It uses Spring Cloud and is not heavy on the application development process.

https://www.tutorialspoint.com/spring_boot/spring_boot_eureka_server.htm#:~:text=Eureka%20Server%20is%20an%20application,also%20known%20as%20Discovery%20Server.