Several directories are always available in Linux because they are used by the system. These are:

Directory Use
/ The "Root" directory
/var Variables
/bin Binaries
/etc System
/sbin Special binaries
/usr Unix system resources
/usr/lib Installed program files
/mnt (or /media) Mounted devices
/dev Devices
/boot Boot loaders, kernel
/lib Libraries
/home Users' home directory

Users cannot write in any directory but their own home-directory (user pietje can only write in /home/pietje). Only root can write in other directories and can give permission to others to write elsewhere. All directories are readable to anyone unless root restricts it.