After the kernel started up，following with file system.

   init is the first user space process spawned by the kernel after completion of the boot process. And init can exist in a single runlevel(7 levels) at any given time.  Init invokes some scripts under a directory called /etc/rc.d/init.d that includes every service script like nfs,syslog. Init also reads the system configuration file /etc/inittab. Refer to man init and man inittab if want details.

    The initrd is a temporary filesystem commonly used in the boot process of the Linux kernel. It is typically used for making preparations before the real root file can be mounted.