The harddrive of a computer is where all of your data is saved. The data includes the system files to run your computer, any programs on your computer, your personal settings, documents and media (pictures, music and videos). Some computers may have more than one harddrive. Harddrive space is measured in gigabytes. Most modern desktop computers have a 250GB harddrive or more, whilst laptops usually have smaller sized harddrives, starting around 100GB. The bigger the number of gigabytes, the better.
You can buy additional harddrives for your desktop or laptop to store extra data. These are called external harddrives, because they connect to your computer by a USB port. Internal harddrives are built into the computer (although most manufacturers allow you to remove them).