Most people have several thousand files on their computer's hard disk, so imagine how hard it would be to find anything if the files were not organized. Fortunately, all hard disks use a file system, which organizes all the files on the disk. The file system is created when you initialize or format your hard disk. It sets up the root directory and subsequent directories beneath it. The file system allows you to create new files and folders, which are added to different parts of the "file tree" on your hard disk.
For example, your hard disk probably has separate folders for programs, documents, pictures, music, and movie files. Within these folders, there are likely other folders that futher organize your files. All these folders (or directories) are organized by your computer's file system. There are also several folders your computer's operating system uses to store system files, such as startup data and system preferences. Some of these folders are invisible to the user, but are recognized by the computer's file system.
Older Windows machines used a file system called FAT32, while newer Windows computers use NTFS. Macintosh computers used the HFS file system for a long time, but now use an updated version of HFS, called HFS+. Though you typically don't need to know all the details of your computer's file system, it is nice to know that it is always working to keep your files organized.