/usr

The /usr directory is a secondary file hierarchy that contains shareable, read-only data.

It includes the following:

/usr/bin/
A directory that contains most user commands.
/usr/include/
The top of a directory structure that contains the C and C++ header files. This directory includes sys, platform-specific, and other directories.
/usr/info/
Documentation for various utilities.
/usr/lib/
Object files, libraries, and internal binaries that you shouldn't execute directly or in scripts. You'll link against these libraries if you write any programs.
/usr/libexec/
A directory that could contain system daemons and system utilities; in general, these are run only by other programs.
/usr/local/
A directory where the system administrator can install software locally. It's initially empty.
/usr/man/
"Manual pages" for various utilities.
/usr/qde/
The top of a directory structure that contains executables, data files, plugins, etc. associated with the Integrated Development Environment (IDE) on Linux and Windows.
/usr/sbin/
Nonessential system binaries, such as dumper , and nicinfo .
/usr/share/
Data that's independent of the architecture, such as icons, backdrops, and various gawk programs.
/usr/src/
A directory for source code.