The root directory contains the entire 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. It contains the basic commands which will be used by all users and which are necessary for the operation of the system: ls, cp, login, etc. Static, unsharable.
/boot: contains the files required by the GNU/Linux boot-loader (GRUB or LILO for Intel, yaboot for PPC, etc). It may or may not contain the kernel, but if the kernel isn't located in this directory then it must be in the root directory. Static, unsharable.
/dev: system device files (dev for DEVices). Some files contained by /dev are mandatory, such as /dev/null, /dev/zero, and /dev/tty. Static, unsharable.
/etc: contains all configuration files specific to the computer. This directory cannot contain binary files. Static, unsharable.
/home: where all the personal directories of the system's users are located. This directory may or may not be shared (some large networks make it shareable via NFS). Your favorite application's (like e-mail readers or browsers) configuration files are located in this directory and start with a period (“.”). For instance, the Mozilla configuration files lie in the .mozilla directory. Variable, shareable.
/lib: it contains libraries which are essential to the system; it also stores kernel modules in the /lib/modules/KERNEL_VERSION subdirectory. It contains all libraries required by the binaries in the /bin and /sbin directories. The optional ld* execution time linker/loader as well as the dynamically-linked C library libc.so must also reside in this directory. Static, unsharable.
/mnt: directory containing the mount points for temporarily-mounted file systems such as /mnt/cdrom, /mnt/floppy, etc. The /mnt directory is also used to mount temporary directories (a USB card will be mounted in /mnt/removable, for instance). Variable, unsharable.
/opt: contains packages not essential for system operation. It is reserved for add-on packages; packages such as Adobe Acrobat Reader are often installed into /opt. The FHS recommends that static files (binaries, libraries, manual pages, etc.) installed in the /opt structure be placed in /opt/package_name and the specific configuration files in /etc/opt.
/sbin: contains system binaries essential for system start-up. Most of these files can only be executed by root. A normal user may run them, but they might not do anything. Static, unsharable.
/tmp: directory intended to contain temporary files which certain programs may create. Variable, unsharable.
/usr: explained in more detail in the section called “/usr: The Big One”. Static, shareable.
/var: location for data which may be modified in real time by programs (such as mail servers, audit programs, print servers, etc.). Variable. Its various subdirectories may be shareable or unsharable.