The Common Data Service for Apps, is a data store that's used by business applications such as Dynamics 365 and was created by Microsoft. Common Data Service for Apps questions on working with data, creating process designer extensions, or creating Windows client applications should use this tag in addition to tags for the business application and/or the framework (ie: .NET SDK).
Common Data Service (CDS) for Apps lets you securely store and manage data that's used by business applications. Data within CDS for Apps is stored within a set of entities. An entity is a set of records used to store data, similar to how a table stores data within a database. CDS for Apps includes a base set of standard entities that cover typical scenarios, but you can also create custom entities specific to your organization and populate them with data using Power Query.
See the Common Data Services for Apps Developer Overview for more information.