Difference Datamodel CDS and Service CDS

Hi all,

made the tutorial "Build Your First OData-based Backend Service" which can be found on

https://developers.sap.com/group.scp-8-odata-service.html

Could anybody please explain me the difference an the linkage between the datamodel CDS and the service CDS?

There are tons of documentations but I cannot find the purpose? Why is that needed?