First Stage Bootloader: Searches, loads and executes the MBR boot loader program.

MBR(Master Boot Record): Located in the 1st sector of the bootable disk. It loads and executes the second stage
bootloader.

Second Stage Bootloader: Displays a splash screen. It loads the default kernel image as specified in the configuration file, has the knowledge of the filesystem. It loads and executes Kernel and initrd images.

Kernel: The kernel that was selected by second stage
bootloader first mounts the root file system that's specified in the config file. Then it executes the /sbin/init program, which is always the first program to be executed. You can confirm this with its process id (PID), which should always be 1.

Init: Identifies the default initlevel from /etc/inittab and uses that to load all appropriate program.

Runlevel programs: When the Linux system is booting up, runlevel programs, executed from the run level directory as defined by your run level.