under the folder /etc/init.d- you will find all the init scripts for different boot up services. like apache2 , networking etc.
depending on which runlevel the computer starts in, different services are started. so from the /etc/init.d/ folder each "service" is linked to one , many , or no run level folders named from rc0.d to rc6.d
to keep things simple there is a tool for removing / adding theese links, hence removing or adding scripts to and from startup.
to remove apache2 simply type:
sudo update-rc.d -f apache2 remove
and all run level folders that have apache2 linked to them will the apache2 service removed
