Floppy Disk / Hard Disk
Several people get these two terms mixed up. "Floppy" and "hard" refer to the magnetic recording material of the disk, NOT the cover! Thus, 5.25" and 3.5" removable diskettes are "floppies", even though 3.5" floppy disks have a sturdy case. (If you opened the cover you could easily tell the difference, but you would also risk damaging the disk.)
You never see the recording material of the hard disk, because the hard disk drive has no openings at all. But trust me, it's a rigid surface! That's why hard disks can 'crash' if jolted too hard; the disk surface can't bend to get out of the way of the read/write head, so the head makes a scratch on the surface.
The term "Compact Disk" actually does refer to its size, even though it's larger than a 3.5" floppy. This is because CD's were not created for computers, but for music, and they are certainly smaller than a standard vinyl record! Just keep in mind that the word "diskette" is always used to refer to floppies, and Compact Disks are usually called "CD's".