/ 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.