/etc/rc or /etc/rc.d or /etc/rc?.d

/etc/rc.d is very important directory, its contain Scripts or directories of scripts to run at startup. These scripts are automatically run at boot time. These files are,

 /etc/rc    
 /etc/rc.conf
/etc/rc.d/*
/etc/rc.lkm
/etc/rc.local
/etc/rc.shutdown
/etc/rc.subr
/etc/defaults/*

/etc/rc :  Will runs the kernel has initialized all devices at startup.

/etc/rc.conf : This configuration file for loads its default settings from /etc/defaults/rc.conf

/etc/rc.d : Sorts the scripts in rc.d using rcoder and runs them in that order.

/etc/rc.lkm : This is for loads or unloads Loadable Kernel Modules  

/etc/rc.shutdown :  will run for shutting down the system.

/etc/rc.subr : It's contains common functions used by /etc/rc and rc.d scripts.

/etc/rc.local : This is almost the last script called at boot up. have permission to edit by the administrator to start local daemons. If you want to start by default can do it.

For example if have enabled the sshd on runlevel

Starting sshd.    [ok]