In a UNIX-based Operating System, /usr is a directory for storing most of the common binaries (and supporting files).
There is a big misconception that the /usr directory stands for "user", when instead it stands for "Unix System Resources" (at least as of base System V and up). If any directory is worthy of the title "user", it would be /home.
Note: ariels made a good point that earlier *NIX-based systems contained the user accounts within the /usr directory, so it is possible to associate "users" (not "user") with /usr. At least until /home was established as a common partition/directory.
The average structure of /usr is as follows. This can be anywhere from slighty to greatly different, depending on a *NIX-distribution:
- /usr/bin: Holds extension (not part of root system) binaries for common usage among users. /bin holds the system-level binaries instead.
- /usr/doc: Holds documentation for applications/programs/binaries for common users to read.
- /usr/etc: Contains any configuration files not used by the root system (such as games, etc).
- /usr/games: Some *NIX distributions contain this directory for game applications.
- /usr/include: The primary include directory for development. Sometimes applications come with interfaces which can be used for writing programs based on the original (i.e. vorbis, lame).
- /usr/lib: This directory hosts the shared-library and object files used for development purposes (with their interfaces in /usr/include).
- /usr/libexec: Common place for executable files added by more popular applications (such as: emacs, awk, openssh, etc). Sometimes these are small utilities that are supplied for entertainment purposes, such as emac's yow binary.
- /usr/local: Many programs install within sub-directories here.
- /usr/sbin: This is the location of the superuser - or root - binaries. These are mostly what is not included in /sbin, which is for system-level binaries.
- /usr/share: This is one of the spotlight directories. All bitmaps, sound files, etc. are put here for all common users to read. For example, most X11 wallpapers are placed within subdirectories of this.
- /usr/src: This houses source code that is usually installed with utilities like RPM.
- /usr/tmp: This is the temporary buffer-like directory that many applications and users write to for one-time usage.
- /usr/X11R6: This directory is called: X11RRevision Number. This has most of the binaries, libraries, documentation, and includes (for development purposes) for running the X11 Graphical Windowing Desktop