Explain Service layer in Hybris multi-channel Suite? what is service layer ?

Service layer is interface implemented class which is abstract from persistence layer which contain function logic and no-persistence related code
So when you writing any service layer make sure that it should loosely coupled with persistence layer as possible .
Hybris multichannel suite expose all functionality in ther form of service
1. Bussiness Service- service such as cart handling , Back order etc
2. Infrastructure service  - service such as internationalization, export service, import service
3. System service - such as model handling , session handling.

Service layer has interacted with persistence layer via DAO layer and the through using Model it has interacted with service layer and DAO has interacted with
persistence layer through Flexible search and SQL statement.