This article by Vijay Narayanan, defines data services as "the encapsulation of operations on key data entities of relevance to the enterprise." He then makes a general case for data services:
Data services abstract the consumer from having to access or update multiple data sources and are critical in helping maintain data integrity when a consumer needs to work with multiple data sources. Additionally, they help build reusable data services that can be leveraged for multiple projects and initiatives. Data services also perform a critical governance function – they help centralize metrics, monitoring, version management, reuse of data types, and enforce data visibility and access rules.
before addressing specific issues of data services.
Several benefits of using data services (source abstraction, aggregation, reuse, and more) are noted and briefly discussed. Scope of data services is defined very narrowly, "actions on data entities-period," along with why this scoping is important.
The article then looks at data services development following a "contract-first" approach and concludes with a discussion of data services consumption patterns.
Read An Introduction to Data Services for more information on this topic.