Root File System (RFS) in Linux

File system: Place to store data in form of files


The root file system refers to the file system mounted at the base of the file system hierarchy 
designated simply as /.

A minimal RFS should contain the following:

        Binaries (/bin,/sbin)
        Libraries (/lib)
        Devices (/dev)
        Configurations (/etc)
        Virtual File Systems (/proc, /sys)
        Applicaiton temporary file (/tmp)
        variable data from daemons & utilities (/var)
        user data (/root, /home) - Optional
        Mount points (/mnt, /media) - optional
        Additional software (/usr,/opt) - optional
        Bootloader files (/boot) - optional


bin - Binary executables, used by all users on the system
dev - Device nodes
etc - local system configuration files
home - user account files
lib - system libraries such as the  C library and many others.
sbin - binary executables usually reserved for superuser accounts on the system.
tmp -temporary files
usr - a secondary file system hierarchy for application programs, usually read-only.
var - contains variable files, such as system logs and temporary configuration files.
proc - a special file system containing system information
mnt - a placeholder for user-mounted devices and file systems.

Links:
1. http://libraryopt.sourceforge.net/
2. http://www.openembedded.org/wiki/Main_Page
3. http://buildroot.uclibc.org/