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, unshareable.
/boot/: contains the files required by the GNU/Linux bootloader (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, unshareable.
/dev/: system device files (dev for DEVices). Static, unshareable.
/etc/: contains all configuration files specific to the computer. Static, unshareable.
/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). Variable, shareable.
/lib/: it contains libraries which are essential to the system; it also stores kernel modules in the /lib/modules/KERNEL_VERSION/ subdirectory. 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 temporarily-mounted file systems. Variable, unshareable.
/opt/: holds packages not essential for system operation. It's recommended that static files (binaries, libraries, manual pages, etc.) to be placed in /opt/package_name and the specific configuration files in /etc/opt/.
/usr/: explained in more detail in the section called “/usr/: The Big One”. Static, shareable.
/sbin/: contains system binaries essential for system start-up. Most of its files can only be executed by root. A normal user may run them, but they won't do anything for a regular user. 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 (such as mail servers, audit programs, print servers, etc.). Variable. Its various subdirectories may be shareable or unshareable.