Source : Free On-Line Dictionary of Computing

USR
     
        {U.S. Robotics, Inc.}

usr
     
        User.  The "/usr" directory hierarchy on {Unix} systems.  Once
        upon a time, in the early days of Unix, this area actually
        held users' home directories and files.  Since these tend to
        expand much faster than system files, /usr would be mounted on
        the biggest disk on the system.  The root directory, "/" in
        contrast, contains only what is needed to {boot} the {kernel},
        after which /usr and other disks could be mounted as part of
        the multi-user start-up process.
     
        /usr has been used as the "everything else" area, with many
        "system" files such as compiler libraries (/usr/include,
        /usr/lib), utilty programs (/usr/bin, /usr/ucb), games
        (/usr/games), local additions (/usr/local), manuals
        (/usr/man), temporary files and queues for various {daemon}s
        (/usr/spool).  These optional extras have grown in size as
        Unix has evolved and disks have dropped in price.  Under later
        versions of {SunOS}, the user files have fled /usr altogether
        for a new "/home" {partition} and temporary files have moved
        to "/var".  This allows /usr to be mounted read-only with some
        gain in security and performance since access times are not
        updated for files on read-only file systems.