What is a hard drive into a computer?

Answer 1

The hard drive is the computer's long-term memory. Our brains use different methods to remember things we want kept for a long time, like our names and the names of our family members, than remembering to carry the one in a math problem. It is effectively the same for computers.

A computer will 'write' things down on the hard drive to remember them long-term. The Random Access Memory(RAM) a computer has is what it would use in the above math example, instead of the hard drive. When your computer is starting up when you first turn it on, it is remembering all the things it 'wrote' down that you want to be able to use, like, say, Windows XP. XP will stay in the computer's RAM while you are logged in and using it, as will any programs you start, like, say, internet Explorer. All these must have been 'written' down first in the hard drive.

The reason I use the quotes around 'write' is that hard drive use actual metal disks (hence them also being called hard disks, or hard disk drives) and they store information by very, very precise use of magnets and magnetic fields.