/usr

• contains all user binaries, their documentation, libraries, header files, etc. - contains only static data (as opposed to /var which contains variable data)
• some interesting subdirectories in /usr :

  • /usr/bin

    contains most binaries on the system (i.e. diff, emacs, gzip, g++, svn, etc.)
    
    

  • /usr/include

    header files for C programming language and header files used by various programs on the system
    
    

  • /usr/lib

    program libraries (collections of frequently used routines) - library of unchanging data files for programs
    
    

  • /usr/local

    locally installed software - reserved for the use of the local admin(s).
    
    

  • /usr/sbin

    system admin commands not needed on the root filesystem (i.e. most server programs such as: chrond, httpd, sshd, etc.)
    
    

  • /usr/share/doc

    miscellaneous documentation files
    
    

  • /usr/share/info

    GNU info documents
    
    

  • /usr/share/man

    manual pages
    
    

  • /usr/src

    a linux or Redhat or some such similar subdirectory contains the Linux kernel source, header files and docs
    
    

  • /usr/X11R6

    files for the X Window System
    
    

see also UnixFileDirectoryOrganization