A computer hard drive is the place where computer data is stored. A HDD (Hard Disk Drive) is generally referred to as the secondary computer storage device, as the RAM (Random Access Memory) is the primary. Data that is stored on the RAM is volatile meaning when the computer is shut down then the information will be lost.
What is a Hard Drive?
When you turn on your computer it starts the Boot Process which tells the computer to go to the hard drive for the Operating System.
A hard drive is a permanent storage device, so even when the computer is turned off the information will still be there. The operating system, program files, and all data files are stored on the hard drive. A computer can function without a hard drive but there would be no operating system or programs so it would be of very little use.
A hard drive uses circular platters to store data. These are kept in pristine condition and are very good mirrors. If you open a hard drive case, the air will contaminate the platters and it is unlikely that it would work again. The actuator arm runs between the platters reading the information in 1's and 0's. The head of this arm reads data even when the disk is running at 7200rpms (a common hard drive speed).
This means that the platter is doing 7200 rounds per minute. More recent computer hard drives can now do up to 15,000rpms. As the platters go round and round the head reads it and processes the information which then proceeds to the connector which connects to the computer.