It's probably a good idea to explain a little theory before discussing the mechanics of using disks. In particular, you must understand the concept of a filesystem. This can be a bit confusing because it has several meanings.
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.