The /etc/rc.d/rc script is the last major step in the startup process. The rc script initializes the run level defined in the /etc/inittab file by running the scripts located in the directory for that run level, i.e., /etc/rc.d/rc3.d for run level 3.
The rc script is fairly short. It first verifies that a valid run level has been passed to it. It runs all of the Kill scripts in that run level and then runs all of the Start scripts in that run level.
Refer to Understanding Run Levels for more information about the run levels themselves.