A hard disk drive (HDD, often just called "hard drive") is a data storage device used for storing and retrieving digital information. An HDD retains its data even when powered off. Data is read in a random-access manner, meaning individual blocks of data can be stored or retrieved in any order rather than just sequentially. An HDD consists of one or more rigid ("hard") rapidly rotating discs (platters) with magnetic heads arranged on a moving actuator arm to read and write data to the surfaces.
Although not originally, the HDD is one now of the core components that make up a PC. The hard disk capacity determines the amount of files and programs that you can store and the disk speed and type have a great influence on the overall performance of the computer.