"HDD" refers to the hard disk drive, which About advises is the main data storage device on a computer. The operating system, application software and most of the other files are stored on the computer's HDD.
The HDD is also often referred to as the C drive. The Microsoft Windows operating system labels the main partition of the drive as the C drive. The physical HDD is typically about the size of a small paperback book, and it is usually mounted with screws in the computer casing. It connects to the motherboard of the computer via a port and to the power supply.