The init script are bash script that are run by the init daemon.
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.