The filesystem refers to the whole directory tree, starting with the root directory /, as described in earlier chapters.
A filesystem in general means any organization of files and directories on a particular physical device. “Organization” means the hierarchical directory structure and any other information about files one might want to keep track of: their size, who has permission to change them, etc. So you might have one filesystem on your hard disk, and another one on each floppy disk.
“Filesystem” is also used to mean a type of filesystem. For example, MS-DOS and Windows 3.1 organize files in a particular way, with particular rules: Filenames can have only eight characters, for example, and no permission information is stored. Linux calls this the msdos filesystem. Linux also has its own filesystem, called the ext2 filesystem (version two of the ext filesystem). You’ll use the ext2 filesystem most of the time unless you’re accessing files from another operating system or have other special needs.
Any physical device you wish to use for storing files must have at least one filesystem on it. This means a filesystem in the second sense—a hierarchy of files and directories, along with information about them. Of course, any filesystem has a type, so the third sense will come into play as well. If you have more than one filesystem on a single device, each filesystem can have a different type—for example, you might have both a DOS partition and a Linux partition on your hard disk.
10.2 mount and /etc/fstab
This section describes how to mount a floppy or Zip disk, discusses the /dev directory, and addresses distributing the directory tree over multiple physical devices or partitions.
10.2.1 Mounting a Filesystem
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.