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 configuration files specific to the computer. Static, unshareable.
/home: contains all the personal directories of the system's users. This directory may or may not be shareable (some large networks make it shareable by NFS). Variable, shareable.
/lib: this directory contains libraries which are essential to the system; it also hosts kernel modules in /lib/modules. All libraries required by the binaries in the /bin and /sbin directories must be located here, together with the ld.so linker. Static, unshareable.
/mnt: directory containing the mount points for temporary file systems. Variable, unshareable.
/opt: it holds packages not required for system operation. It is recommended to place static files (binaries, libraries, manual pages, etc.) for such packages in /opt/package_name and their specific configuration files for the computer in /etc/opt.
/root: home directory for root. Variable, unshareable.
/usr: see next section. Static, shareable.
/sbin: contains system binaries essential to the system startup, operable only by root. 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 (i.e.: e-mail server, audit programs, print server, etc.). All of /var is variable, but its different subdirectories may be shareable or unshareable.