A hard disk drive is a non-volatile storage device which stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Hard disks record data by magnetising a magnetic material in a pattern that represents the data. They read the data back by detecting the magnetization of the material.
The hard disk consists of a spindle which holds one or more flat circular disks called platters, on which the data is recorded. The platters are made from a non-magnetic material and are coated with a thin layer of magnetic material.
The platters revolve at very high speeds and information is written to platters as they rotate past read-and-write heads that operate very close over the magnetic surface. The read-and-write heads detect and modify the magnetization of the material immediately under it. There is one head for each magnetic platter surface on the spindle, mounted on a common arm. An actuator arm moves the heads roughly radially across the platters as they revolve, allowing each head to access the entire surface of the platter as it revolves.
Hard disks have sealed enclosure to protects the internal parts from dust, moisture, condensation, and other sources of contamination. The gap between read-write head and platter is only a few nanometers. Most Hard disks now revolve at 5,400 RPM (Revolutions Per Minute) and above.
Defragmentation of Hard Disk, a utility available in Windows (Start -> Programs -> System Tools ->Disk Defragmenter), once or twice a year increases the efficiency of the Hard Disk by reducing the seek time and results in the computer becoming faster. It is generally a good idea to remove programs/utilities, you no longer intend to use, to free system resources.