On a GNU/Linux system there’s no necessary correspondence between
directories and physical devices as there is in Windows, in which
each drive has its own directory tree beginning with a letter (such
as C:\).
Instead, each physical device such as a hard disk or floppy disk has
one or more filesystems on it. In order to make a filesystem
accessible, it’s assigned to a particular directory in another
filesystem. To avoid circularity, the root filesystem (which
contains the root directory /) is not stored within any other
filesystem. You have access to it automatically when you boot
Debian.
A directory in one filesystem that contains another filesystem is
known as a _mount point_. A mount point is a directory in a first
filesystem on one device (such as your hard disk) that “contains” a
second filesystem, perhaps on another device (such as a floppy
disk). To access a filesystem, you must mount it at some mount
point.
So, for example, you might mount a CD at the mount point /cdrom.
This means that if you look in the directory /cdrom, you’ll see the
contents of the CD. The /cdrom directory itself is actually on your
hard disk. For all practical purposes, the contents of the CD become
a part of the root filesystem, and when you type commands and use
programs, it doesn’t make any difference what the actual physical
location of the files is. You could have created a directory on your
hard disk called /cdrom and put some files in it, and everything
would behave in exactly the same way. Once you mount a filesystem,
there’s no need to pay any attention to physical devices.
However, before you can mount a filesystem or actually create a
filesystem on a disk that doesn’t have one yet, it’s necessary to
refer to the devices themselves. All devices have names, which are
located in the /dev directory. If you type ls /dev now, you’ll see a
pretty lengthy list of every possible device you could have on your
Debian system. For a summary of some devices, see Table 2.1 on page
[*]. A more thorough list can be found on your system in the file
/usr/src/linux/Documentation/devices.txt.
To mount a filesystem, we want to tell Linux to associate whatever
filesystem it finds on a particular device with a particular mount
point. In the process, we might have to tell Linux what kind of
filesystem to look for.