Several directories are always available in Linux because they are used by the system. These are:
|/||The "Root" directory|
|/usr||Unix system resources|
|/mnt (or /media)||Mounted devices|
|/boot||Boot loaders, kernel|
|/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.