Referrence <SAP NetWeave Roadmap>
Service
A service is a function in program.
Web Service
A web service is a service that is available through use of internet technology and that can be addressed externally.
It communicates using open standards and has its own logic for addressing the underlying application.In most cases,it also
has its own data retention.
Application service
An application service is a web service that is offered by an ERP application.The term "application service" is used primarily
in the context of SAP ERP and non-ERP systems(such as CRM).
Enterpise service
An Enterprise Service groups a number of application services together to create a logical unit,such as the "business partner" object.
An enterprise service contains only the logic,but dosen't have its own data retention,which remains in the individual subsystems.
Composite Applications
Composite applications are building block applications created using Web service.Composite applications can use services
from multiple existing applications.The hub groups different application service into enterprise services, and composite
applications can the use these enterprise services to carry out the nescessary work.
Package Commposite Applications
The term packaged composite applications generally refers to composite applications pacakged and developed by software manufacturers
or specific companies.Packaged composite applications can be created individually.
Cross-applications (xApps)
Cross-applications,or xApps in the SAP context,are applications that are more than mere enterprise services. While they use the same
logic,they have a more powerful scope,and deffer from applications that are wholly implemented in a single system.An xApp is
a composite application that SAP has filled with content.