In addition to what fow99 said, it is the alternative to a "fat32" or "fat16" file system. i.e. All Win95 and Win98 use fat32 (or sometimes fat16) file systems based on a fat (file allocation table). I'm not sure how NTFS works, but supposedly it is more efficient and faster than a fat system. As far as I know, NTFS is an option only on NT or Win2K systems.