The init script are bash script that are run by the init daemon.
see Init - How to develop a SysV Init Script
See Linux - (init.d|Initialization) Location
In order to control any of the scripts in init.d manually you have to have root (or sudo) access.
Each script will be run as a command and the structure of the command will look like:
/etc/init.d/command OPTION
Where command is the actual command to run and OPTION can be one of the following:
For instance:
/etc/init.d/httpd stop
/etc/init.d/networking restart
Note that traditional init scripts continue to function on a systemd system.
An init script /etc/rc.d/init.d/foobar is implicitly mapped into a service unit foobar.service during system initialization.