Each device listed under the <gui>File Systems</gui> tab is a storage disk (like a hard drive or USB memory stick), or a disk partition. For each device, you can see what its total capacity is, how much of its capacity has been used, and some technical information about what <link xref="fs-info">type of filesystem it is</link> and <link xref="fs-info">where it is "mounted"</link>.
The disk space on a single physical hard disk can be split up into multiple chunks, called partitions, each of which can be used as if it were a separate disk. If your hard disk has been partitioned (perhaps by you or the computer manufacturer), each partition will be listed separately in the File Systems list.