ESR is a meta data repository, meaning that it stores data that describes other data. The meta data inside the ESR is then used by development tools, modeling tools, operational management tools, and by other services to help them do their jobs. The ESR has also been given the job of storing descriptions of business objects and models that show how services work together in process components.
Perhaps the best way to quickly understand the ESR is to list all the different types of metadata stored in in and quickly explain how that data is used.
Type of data and How it is used as follows:
Web Services Definition Language (WSDL) files These files describe the service interfaces of enterprise services and are used by development tools and during run-time to look up interface definitions.
Business Object Descriptions Business objects are used in modeling to show what structures exist in an application that are implementing services.
Process Component Models This data describes models that show how enterprise services work together to implement business processes. These models show how a service is used and what business objects implement the service.
It is important to remember that services are not implemented in the ESR. The ESR holds descriptions of services that are then implemented in applications that are separate from the repository
In PI 7.1 ( the next release of PI ) the IR of PI will become the Enterprise Service Repositroy.
Take a look at these articles,