What is a filesystem?

Before delving deeply into such topics as inodes and attributes structures and symbolic links, let's step back a little and discuss what a filesystem is. At the most basic level, a filesystem is usually a hierarchical arrangement of data. I say "usually" because several (historical) filesystems were flat filesystems—there was no hierarchical ordering of the data. An example of this is the old floppy-based MS-DOS 1.00 and 1.10 filesystems. A nonhierarchical filesystem is a subset of a hierarchical one.