A hard drive is an essential piece of hardware for every computer, it actual name is Hard Disk Drive (HDD), other more common names are hard disk and fixed disk. A hard drive is a non-volatile data storage device; using two rotating disks, known as platters, that have magnetic surfaces, the hard drive digitally stores encoded data. While modern day Hard drives are a completely sealed unit (aside from the ventilation hole), the original hard drives had removable media. The term "Hard" was meant to be a substitute, temporary slang, for the term "rigid" before a universal name had been agreed upon. A rigid-disk drive is a hard drive essentially, but that name is rarely used.

Hard Drives were first introduced for data storage in an accounting computer in 1956 and were developed for general purpose computers. Non-volatile storage means that unlike RAM, which is volatile memory, the information will not be deleted when the computer is powered off. The hard drive is where firmware and program files, along with other types of information, are stored so it can be accessed at any time.

The data is stored in the computer language known as binary code; information is recorded as a series of zeros and ones and is retrieved by detecting the magnetized material. Covered in a thin layer of magnetic material, the platters themselves are non-magnetic usually made of aluminum or glass. Earlier hard drives used iron oxide for the magnetic material, but now a cobalt-based alloy is used.

A typical hard drive for a desktop computer holds somewhere between 120 and 500 GB of digital data, rotate at 7200 RPM (Rotations Per Minute) and transfers 1 Gbit per second. As of July 2008 the largest capacity of a hard drive became 1.5 TB, with an RPM of 10 thousand or 15 thousand and has a transfer rate of 1.6 Gbit per second. Hard drives with RPMs of at least 10 thousand are the gamers' choice for their computers because of the high data transfer rate, which cuts down on the game start up time and decreases game loading time.

Modern day hard drives cover a variety of digital applications, such as video recorders, audio players, personal assistants, cameras and video games consoles. Samsung and Nokia added hard drives to their phones for the first time in 2005 and revolutionized the market. The size of the hard drive stays the same, but the amount of data that can be stored upon them is ever increasing because of data compression research. The original hard disk drive held 4.4 megabytes (MB) of digital data as compared to the newest hard drive today that can hold up to the 3 terabytes (TB) of digital data. (1 thousand bytes = 1 MB; 1 thousand MB = 1 gigabyte (GB); 1 thousand GB = 1 TB) The 3 TB hard drives are not yet available for public purchase but the 2 TB external hard drives are, and they sell for a minimum of $454.