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 structured, 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.