Service vs Listener in oracle
The Oracle Net Listener


  The Oracle Net Listener (the listener) is a server-side process that
  listens for incoming client connection requests and manages traffic to
  the database.


Service Names


  A service name is a logical representation of a service used for
  client connections.
  
  When a client connects to a listener, it requests a connection to a
  service. When a database instance starts, it registers itself with a
  listener as providing one or more services by name. Thus, the listener
  acts as a mediator between the client and instances and routes the
  connection request to the right place.
  
  A single service, as known by a listener, can identify one or more
  database instances. Also, a single database instance can register one
  or more services with a listener. Clients connecting to a service need
  not specify which instance they require.