From the SAP Terminology Guide:
Enterprise services are "a standards-based way of encapsulating enterprise functionality and exposing it as a reusable business service that can be combined with other services to meet new requirements. Enterprise services, defined by SAP and its partners and customers, can be assembled together to compose new applications or enable new business processes." (Source: SAP)
Enterprise services are SAP's term for services that have the proper scope to play a productive role in automating business processes in enterprise computing. The scope of an enterprise services is important because if too much functionality is included in a service, it becomes complicated to reuse. If too little functionality is included then the services are like small utilities and the applications that use them become complicated. Designing services that are the right scope is an engineering art. The Enterprise Services Design Guide describes SAP's approach to services design.
Enterprise services are implemented as web services, meaning that their interfaces are defined using WSDL files and that they support other commonly used standards for web services.
Descriptions of enterprise services, including the WSDL files and process component models that show how services work together to automate processes and business scenarios, are contained in Enterprise Services Repository, a design-time repository of meta-data about enterprise services that is used by development tools.
There are generally two approaches to designing services. Designing services from the outside-in means analyzing the business process that is going to be automated and then constructing definitions of the services. Designing services from the inside-out means that you are creating a service from existing functionality. SAP enterprise services development tools support both approaches and both are needed in any program of service enablement.