The Hard Disk is the place in your computer where data is stored long term. The name 'hard disk' was adopted to differentiate it from the 'floppy disk' (which in the early days really was floppy!).
The hard disk in the pictures below has had the casing removed so you can see inside. The hard disk pictured is a Hewlett Packard with a storage capacity of 340Mb which was probably state of the art around 1992 / 93.
Today's typical hard disk has a storage capacity of 500Gb - that's well more than 1500 times more storage than back in 1992/93.
The interesting thing is that the physical dimensions have not changed despite the overwhelming increase in storage.
What you can just see in the pictures to the above are two shiny discs (called "platters"), one sitting on top of the other. Hard disks may have two or more shiny discs (referred to as "platters"). The device that sits above the disk is the arm, on the tip of which is an electronic device that can either impart or receive electrical charges to and from the magnetic surface of the disk. Since there are two platters in this hard disk, there are in fact two arms (one for each platter).
When in operation the arm moves across the surface extremely fast. It works so hard that eventually, probably between 3-5 years it begins to malfunction and cause micro damage to the surface of the disks. Then your hard disk begins to report "bad sectors".
Hard drives for laptop computers are much smaller and moe susceptible to damage due to the nature of laptops and notebook computers being frequently moved around. Vibration caused while the computer is actually in operation is not good for the hard disk. Therefore shutdown your laptops before you move them.