The technical description of a hard drive would probably go something like this - a collection of hard platters coated with magnetic material to which data can be written and read using a series of read/write heads.

A drive might have up to eight platters (although the current trend is to use a smaller number of larger platters) which rotate, typically, at speeds of 5420 or 7200 rpm. The whole unit is sealed inside a case which prevents dust getting inside. The heads fly above the platters at a distance of from 10 to 25 millionths of an inch and a speck of dust could cause serious damage.

Hard drives have been getting cheaper and their storage capacity larger over the years. Rare now is the PC which comes with a drive smaller than 80Gb (other than laptops), and 250Gb and 500Gb drives are the norm.

Modern drives connect to the motherboard via a SATA interface. The eSATA interface was designed for adding an external drive and there are also very affordable USB drives if you need additional external storage.