Pour connaitre la hierachie standard Ubuntu :

man hier

Les plus importants :

    /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 holds only the files which are needed during the boot process.
            The map installer and configuration files should go to /sbin and /etc.  The operating system kernel (initrd for  example)
            must be located in either / or /boot.

     /dev   Special or device files, which refer to physical devices.  See mknod(1).

     /etc   Contains configuration files which are local to the machine.  Some larger software packages, like X11, can have their own
            subdirectories 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.
     /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 (optional).

     /lib   This directory should hold those shared libraries that are necessary to boot the system and to run the  commands  in  the
            root filesystem.

     /media This  directory  contains mount points for removable media such as CD and DVD disks or USB sticks.  On systems where more
            than one device exists for mounting a certain type of media, mount directories can be created by appending a digit to the
            name of those available above starting with '0', but the unqualified name must also exist.
     /mnt   This  directory  is a mount point for a temporarily mounted filesystem.  In some distributions, /mnt contains subdirecto‐
            ries intended to be used as mount points for several temporary 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-filesystem 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 system, but which are usually not executed by normal users.

     /srv   This directory contains site-specific data that is served by this system.

     /sys   This is a mount point for the sysfs filesystem, which provides information about the kernel like /proc, but better struc‐
            tured, following the formalism of kobject infrastructure.

     /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 shareable, read-only data, so that it
            can be mounted by various machines running Linux.

    /usr/bin
            This is the primary directory for executable programs.  Most programs executed by normal users which are not  needed  for
            booting or for repairing the system and which are not installed locally should be placed in this directory.

     /usr/lib
            Object libraries, including dynamic libraries, plus some executables which usually are not invoked directly. More complicated programs may have whole subdirectories there.
            
                  /usr/local
            This is where programs which are local to the site typically go.
    /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.
     /var   This directory contains files which may change in size, such as spool and log files.