The /usr filesystem
The /usr filesystem hierarchy contains system utilities and programs that do not appear in the root partition. For example, user programs such as less and tail are found in /usr/bin. /usr/sbin contains system administration commands such as adduser and traceroute, and a number of daemons needed only on a normally operating system. No host-specific or variable data is stored in /usr. Also disallowed is the placement of directories directly under /usr for large software packages. An exception to this rule is made for X11, which has a strong precedent for this location.
The following subdirectories may be found under /usr:
This directory contains files for XFree86. Because X is deployed directly under /usr on many Unix systems, X breaks the rule that usually prohibits a custom /usr directory for a software package.
The /usr/bin directory is the primary location for user commands that are not considered essential for emergency system maintenance (and thus are stored here rather than in /bin).
/usr/include is the standard location for include or header files, used for C and C++ programming.
This directory contains shared libraries that support various programs. FHS also allows the creation of software-specific directories here. For example, /usr/lib/perl5 contains the standard library of Perl modules that implement programming functions in that language.
/usr/local is the top level of another hierarchy of binary files, intended ...