A disk drive is used to store programs and data so that they can be reaccessed after a computer has been switched off and then back on. It is the primary method of information storage for most computers. If it is not possible to store the programs and data that is being processed then they will have to be reloaded each time they are required from some other method of storage. In the past paper tape and punched cards were used, more recently the internet is being used for additional storage. Disk drives are still used on the internet to store the information, however where and what they are is hidden from the surfer.
A disk drive can be a hard disk drive HDD or an optical disc drive ODD. The ODD is usually referred to as a compact disc CD. The floppy disk drive FDD was very popular for storing smaller items of data however this is now old technology and is largely being replaced by memory sticks.
The drive refers to the electrical motor and the mechanical method used to digitally encode data onto a surface. There is the motor that turns the disk and the motor that moves the read write heads to the desired position on the disk. A stepping motor was previously used to move the heads to the correct position now a voice coil actuator tends to be used.
A modern HDD is a device that stores digitally encoded data on high speed rotating rigid platters with magnetic surfaces in a sealed unit. Some disk drives have a small filtered vent hole to equalize air pressure.
The older HDD were physically removable and great care had to be taken to ensure that the computer room air was very clean so that no dust got into the drive. The disk drive magnetic read write heads travel very close to the magnetic surface and any particles of dust would cause the heads to crash onto the surface physically damaging the surface and destroying the data.