A Database Service represents a single database, either a single-instance database or a Real Application Cluster database.
A connection request typically specifies a database service name. If no such name is specified, the default service name found in the listener.ora file is used.
Edition attribute
A service can be given an edition attribute which causes subsequent sessions connecting with this service to use this edition.
Internal services
Oracle maintains two internal services:
SYS$BACKGROUND and
SYS$USERS
These services are used by background processes and user sessions that are not associated with a service.
Global Database Service
A global database service is a service provided by multiple databases synchronized through data replication.
TODO
Services (or service names) are an important aspect when creating PDBs.