What is a service in linux ?

1. A Linux service is an application (or set of applications) that runs in the background waiting to be used, or carrying out essential tasks.
2. Each service has its corresponding service script, these services being managed by shell scripts so called init scripts or service scripts.
3. Normally the name of the scirpt is same as the name of the daemon which script starts when it is executed, for example smb is service if you start this service it will start the daemons smbd and nmbd.

As i told above when you start a service it starts a daemon with the name of its "script name ", and these services are started by executing there corresponding scripts present in the /etc/ininit.d/ directory.