Pour connaitre la hierachie standard Ubuntu :
man hier
Les plus importants :
This directory contains executable programs which are needed in single user mode and to bring the system up or repair it.
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.
Special or device files, which refer to physical devices. See mknod(1).
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.
Host-specific configuration files for add-on applications installed in /opt.
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).
This directory should hold those shared libraries that are necessary to boot the system and to run the commands in the root filesystem.
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.
This directory is a mount point for a temporarily mounted filesystem. In some distributions, /mnt contains subdirectories intended to be used as mount points for several temporary filesystems.
This directory should contain add-on packages that contain static files.
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).
This directory is usually the home directory for the root user (optional).
Like /bin, this directory holds commands needed to boot the system, but which are usually not executed by normal users.
This directory contains site-specific data that is served by this system.
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.
Object libraries, including dynamic libraries, plus some executables which usually are not invoked directly. More complicated programs may have whole subdirectories there.
This is where programs which are local to the site typically go.
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.
This directory contains files which may change in size, such as spool and log files.