/             This directory is called superuser home directory.It is the top of file system structure.All other directory are mounted under it.
 

/boot      This directory contain the kernal image file.This also contain the files related to booting the system such as bootloader.
 

/etc         It contain the whole system configuration file.
 

/home    This is the all users home directory.
 

/mnt       This is a generic mount point under which you mount your filesystems such as cdrom,floppy.
 

/proc      It is not a real file system, it is a virtual file system.This directory is empty until the proc  file system is mounted.
 

/sys        Modern Linux distributions include a /sys directory as a virtual filesystem (sysfs, comparable to /proc, which is a procfs), which stores and allows modification of the devices connected to the system.
 

/dev       This directory contain the devices nodes through which the operating system can access hardware and software devices on the system.
 

/bin        This directory contain the command used by superuser and normal user.
 

/sbin      This directory contain the command used by superuser only.
 

/lib         It contain the library required for running the additional application and running the linux kernal.It also contain the library essential for                   binaries in /bin and /sbin.