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

It includes the following:

A directory that contains most user commands. Examples include diff, errno, and wc.
The top of a directory structure that contains the C and C++ header files. This directory includes sys, platform-specific, and other directories.
Documentation for various utilities.
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.
A directory that could contain system daemons and system utilities; in general, these are run only by other programs.
A directory where the system administrator can install software locally. It's initially empty.
"Manual pages" for various utilities.
The top of a directory structure that contains executables, data files, plugins, etc. associated with the Integrated Development Environment (IDE), which is shipped as part of the QNX Momentics Tool Suite on Linux and Windows.
Nonessential system binaries, such as cron, dumper, and nicinfo.
Data that's independent of the architecture, such as icons, backdrops, and various gawk programs.
A directory for source code.