Hard disks (or hard drive) are secondary storage of computer storage mediums.
A hard disk is consisted of a spinning disk or disks having a single axis.
An electric motor provides the necessary spinning force.
Spinning disks are covered with a thin layer capable of becoming magnetized.
A moving read\write head is responsible for data read and write on surface of disks.
All hard disks have electronics (controllers and cache memory) in addition to mechanical parts mentioned above.
All components of a hard disk are placed in a metal case.
Spinning speed of disks and amount of cache memory are two main factors determining the performance of a hard disk.