.

Description of the file system hierarchy

/      This  is  the  root  directory.   This  is  where the whole tree
       starts.
/bin   This directory contains executable programs which are needed  in
       single user mode and to bring the system up or repair it.
/boot  Contains  static files for  the boot loader. This directory only
       holds the files  which are  needed during  the boot process. The
       map  installer and  configuration  files should go  to /sbin and
       /etc.
/dev   Special or device files, which refer to physical  devices.   See
       mknod(1).
/dos   If both MS-DOS and Linux are run on one computer, this is a typ-
       ical place to mount a DOS file system.
/etc   Contains configuration files which are  local  to  the  machine.
       Some larger software packages, like X11, can have their own sub-
       directories below /etc.  Site-wide configuration  files  may  be
       placed  here  or  in  /usr/etc.   Nevertheless,  programs should
       always look for these files in /etc and you may have  links  for
       these files to /usr/etc.
/etc/opt
       Host-specific   configuration   files  for  add-on  applications 
       installed in /opt.
/etc/sgml
       This directory contains the configuration files for SGML and XML
       (optional).
/etc/skel
       When  a  new  user account is created, files from this directory
       are usually copied into the user's home directory.
/etc/X11
       Configuration files for the X11 window system (optional).
/home  On machines with home directories for users, these  are  usually
       beneath  this directory, directly or not.  The structure of this
       directory depends on local administration decisions.
/lib   This directory should hold those shared libraries that are  nec-
       essary  to  boot  the system and to run the commands in the root
       filesystem.
/mnt   This directory contains mount  points  for  temporarily  mounted
       filesystems.
/opt   This  directory  should  contain  add-on  packages  that contain
       static files.
/proc  This is a mount point for the proc  filesystem,  which  provides
       information  about  running  processes  and  the  kernel.   This
       pseudo-file system is described in more detail in proc(5).
/root  This directory is usually the home directory for the  root  user
      (optional).
/sbin  Like /bin, this directory holds commands needed to boot the sys-
       tem, but which are usually not executed by normal users.
/tmp   This directory contains temporary files  which  may  be  deleted
       with no notice, such as by a regular job or at system boot up.
/usr   This directory is usually mounted from a separate partition.  It
       should hold only sharable, read-only data, so  that  it  can  be
       mounted by various machines running Linux.
/usr/X11R6
       The X-Window system, version 11 release 6 (optional).
/usr/X11R6/bin
       Binaries which belong to the X-Windows system; often, there is a
       symbolic link from the more traditional /usr/bin/X11 to here.
/usr/X11R6/lib
       Data files associated with the X-Windows system.
/usr/X11R6/lib/X11
       These contain miscellaneous files needed to run X;  Often, there
       is a symbolic link from /usr/lib/X11 to this directory.
/usr/X11R6/include/X11
       Contains  include  files needed for compiling programs using the
       X11 window  system.   Often,  there  is  a  symbolic  link  from
       /usr/include/X11 to this directory.
/usr/bin
       This  is  the  primary  directory for executable programs.  Most
       programs executed by normal users which are not needed for boot-
       ing  or  for  repairing  the  system and which are not installed
       locally should be placed in this directory.
/usr/bin/X11
       is the traditional place to look for X11 executables; on  Linux,
       it usually is a symbolic link to /usr/X11R6/bin.
/usr/dict
       Replaced by /usr/share/dict.
/usr/doc
       Replaced by /usr/share/doc.
/usr/etc
       Site-wide  configuration  files  to  be  shared  between several
       machines may be stored in  this  directory.   However,  commands
       should  always  reference  those files using the /etc directory.
       Links from files in /etc should point to the  appropriate  files
       in /usr/etc.
/usr/games
       Binaries for games and educational programs (optional).
/usr/include
       Include files for the C compiler.
/usr/include/X11
       Include files for the C compiler and the X-Windows system.  This
       is usually a symbolic link to /usr/X11R6/include/X11.
/usr/include/asm
       Include files which declare some assembler functions.  This used
       to be a symbolic link to /usr/src/linux/include/asm.
/usr/include/linux
       This  contains  information which may change from system release
       to  system  release  and  used  to  be  a   symbolic   link   to
       /usr/src/linux/include/linux to get at operating system specific
       information.
       (Note that one should have include files there  that  work  cor-
       rectly  with  the current libc and in user space. However, Linux
       kernel source is not designed to be used with user programs  and
       does  not know anything about the libc you are using. It is very
       likely that things will break if you  let  /usr/include/asm  and
       /usr/include/linux point at a random kernel tree. Debian systems
       don't do this and use headers from a known good kernel  version,
       provided in the libc*-dev package.)
/usr/include/g++
       Include files to use with the GNU C++ compiler.
/usr/lib
       Object  libraries,  including  dynamic libraries, plus some exe-
       cutables which usually are not invoked directly.   More  compli-
       cated programs may have whole subdirectories there.
/usr/lib/X11
       The  usual  place for data files associated with X programs, and
       configuration files for the X system itself.  On Linux, it  usu-
       ally is a symbolic link to /usr/X11R6/lib/X11.
/usr/lib/gcc-lib
       contains  executables  and include files for the GNU C compiler,
       gcc(1).
/usr/lib/groff
       Files for the GNU groff document formatting system.
/usr/lib/uucp
       Files for uucp(1).
/usr/local
       This is where programs which are local to the site typically go.
/usr/local/bin
       Binaries for programs local to the site.
/usr/local/doc
       Local documentation.
/usr/local/etc
       Configuration  files associated with locally installed programs.
/usr/local/games
       Binaries for locally installed games.
/usr/local/lib
       Files associated with locally installed programs.
/usr/local/include
       Header files for the local C compiler.
/usr/local/info
       Info pages associated with locally installed programs.
/usr/local/man
       Man pages associated with locally installed programs.
/usr/local/sbin
       Locally installed programs for system administration.
/usr/local/share
       Local application data that can be shared among different archi-
       tectures of the same OS.
/usr/local/src
       Source code for locally installed software.
/usr/man
       Replaced by /usr/share/man.
/usr/sbin
       This  directory contains program binaries for system administra-
       tion which are not essential for the boot process, for  mounting
       /usr, or for system repair.
/usr/share
       This directory contains subdirectories with specific application
       data, that can be shared among different  architectures  of  the
       same  OS.   Often  one  finds  stuff  here  that used to live in
       /usr/doc or /usr/lib or /usr/man.
/usr/share/dict
       Contains the word lists used by spell checkers.
/usr/share/doc
       Documentation about installed programs.
/usr/share/games
       Static data files for games in /usr/games.
/usr/share/info
       Info pages go here.
/usr/share/locale
       Locale information goes here.
/usr/share/man
       Manpages go here in subdirectories according  to  the  man  page
       sections.
/usr/share/man/man[1-9]
       These  directories  contain manual pages for the specific locale
       in source code form. Systems which use  a  unique  language  and
       code set for all manual pages may omit the  substring.
/usr/share/misc
       Miscellaneous  data that can be shared among different architec-
       tures of the same OS.
/usr/share/nls
       The message catalogs for native language support go here.
/usr/share/sgml
       Files for SGML and XML.
/usr/share/terminfo
       The datebase for terminfo.

/usr/share/tmac   
       Troff macros that are not distributed with groff.

/usr/share/zoneinfo
       Files for timezone information.

/usr/src
       Source files for different parts of the  system,  included  with
       some  packages for reference purposes. Don't work here with your
       own projects, as files below /usr  should  be  read-only  except
       when installing software.

/usr/src/linux
       This was the traditional place for the kernel source.  Some dis-
       tributions put here the source for the default kernel they ship.
       You should probably use another directory when building your own
       kernel.

/usr/tmp   
       Obsolete. This should be a  link  to  /var/tmp.   This  link  is
       present only for compatibility reasons and shouldn't be used.

/var   This  directory contains files which may change in size, such as
       spool and log files.

/var/adm
       This directory is superseded by /var/log and should  be  a  sym-
       bolic link to /var/log.

/var/backups   
       Reserved for historical reasons.

/var/cache
       Data cached for programs.

/var/catman/cat[1-9] or /var/cache/man/cat[1-9]
       These directories contain preformatted manual pages according to
       their man page section. (The use of preformatted manual pages is
       deprecated.)

/var/cron
       Reserved for historical reasons.

/var/lib
       Variable state information for programs.

/var/local
       Variable data for /usr/local.

/var/lock
       Lock  files are placed in this directory.  The naming convention
       for device lock files is LCK..  where  is  the  device's name in
       the filesystem.  The format used  is that of HDU UUCP lock files,
       i.e.  lock files  contain  a  PID  as  a  10-byte  ASCII  decimal
       number, followed by a newline character.

/var/log
       Miscellaneous log files.

/var/opt
       Variable data for /opt.
  
/var/mail
       Users' mailboxes. Replaces /var/spool/mail.

/var/msgs
       Reserved for historical reasons.

/var/preserve
       Reserved for historical reasons.

/var/run
       Run-time  variable files, like files holding process identifiers
       (PIDs) and logged user information (utmp).  Files in this direc-
       tory are usually cleared when the system boots.

/var/spool
       Spooled (or queued) files for various programs.

/var/spool/at
       Spooled jobs for at(1).

/var/spool/cron
       Spooled jobs for cron(1).

/var/spool/lpd
       Spooled files for printing.

/var/spool/mail
       Replaced by /var/mail.

/var/spool/mqueue
       Queued outgoing mail.

/var/spool/news
       Spool directory for news.

/var/spool/rwho
       Spooled files for rwhod(8).

/var/spool/smail
       Spooled files for the smail(1) mail delivery program.

/var/spool/uucp
       Spooled files for uucp(1).

/var/tmp
       Like  /tmp,  this  directory holds temporary files stored for an
       unspecified duration.

/var/yp
       Database files for NIS.
  
CONFORMS TO
       The  Filesystem  Hierarchy  Standard,  Version  2.2
       name.com/fhs/

BUGS
       This  list is not exhaustive; different systems may be configured
       differently.

Taken from the manpage of hier ("man hier")