The root directory contains the whole system hierarchy. It cannot be classified since its subdirectories may or may not be static or shareable. Here is a list of the main directories and subdirectories, with their classifications:
/bin: essential binary files. This directory contains the basic commands which will be used by all users and are necessary to the operation of the system: ls, cp, login, etc. Static, unshareable.
/boot: contains the files required by the GNU/Linux boot manager (GRUB or LILO for Intel platforms, SILO for Sparc). This may or may not contain the kernel: if it is not here, it must be located in the root directory. Static, unshareable.
/dev: system device files (dev for DEVices). Static, unshareable.
/etc: this directory contains all the configuration files specific to the machine. Static, unshareable.
/home: contains all the personal directories of users of the system. This directory may or may not be shareable (some large networks make it shareable by NFS). Variable, shareable.
/lib: this directory contains the libraries essential to the system and the kernel modules, in /lib/modules. All the libraries required by the binaries in directories /bin and /sbin must be located here, together with the linker ld.so. Static, unshareable.
/mnt: directory containing the mount points for the temporary file systems. Variable, unshareable.
/opt: this directory contains packages not required for system operation. It is recommended to place static files (binaries, libraries, manual pages, etc.) for such packets in /opt/package_name and their specific configuration files for the machine in /etc/opt.
/root: home directory for root. Variable, unshareable.
/usr: see next section. Static, shareable.
/sbin: contains the system binaries essential to system startup, operable by root only. A normal user can also run them but will not get very far. Static, unshareable.
/tmp: directory intended to contain temporary files which certain programs may create. Variable, unshareable.
/var: location for data which may be modified in real time by programs (e.g. the e-mail server, audit programs, the print server, etc.). All of /var is variable, but its different subdirectories may be shareable or unshareable.