The hard disk drive, also called the hard disk, hard drive, disk drive, and abbreviated HDD contains all saved information in a computer system. All software, documents, and user data is stored on the hard drive. A hard drive is a mechanical device that consists of a series of round, magnetic disks - called platters - that rotate within the enclosure. Next, there are a series of read/write heads for each platter that will read or write a magnetic charge at a specific location on the platter. The rpm (rotations per minute) specification of a hard drive describes how fast these platters can spin. The higher the number, the faster a hard drive is capable of reading or writing data. Because of the use of mechanical systems, a hard drive is much slower than memory, which stores data using electrical charge. The mechanical nature of a hard drive also makes it susceptible to damage if the device is dropped, as the read/write heads are tiny and can easily break or scratch the platters. Damage to the platters can also occur if the computer is turned off while a read or write operation is occurring in a hard drive, or if the drive is moved around during operation. The magnetic storage method can also make the drive vulnerable to data loss if it is put into a strong magnetic field. For this reason, it is always important to back up vital data into more reliable mediums, such as magnetic tape, or even CDs or DVDs.

The size capacity of modern hard disk drives are given in gigabytes, although some measurements may be in megabytes (1 gigabyte = 1024 megabytes), and as hard drives increase in capacity, even in terabytes (1 terabyte = 1024 gigabytes). However, since it is often more convenient for mathematical and marketing purposes, 1000 is often used instead of 1024. This, of course, makes drives seem larger than they really are. A 200GB drive, for example, is usually approximately 186GB. Larger hard drives are useful when a lot of information will be stored on a computer, or if a great number of programs are to be installed. Large hard drives are also useful when archiving music into mp3s, or when storing or recording television or movies onto a computer system.

As is true with most other computer components, hard disk drives come in several flavors. Interface refers to the connection type used to connect the drive to the system board. In all generations of modern hard drives, there are only three different types of interfaces: ATA, SCSI, and SATA.

ATA, or advanced technology attachment, is the standard interface on a system board that is able to accommodate either hard disk drives, or optical drives such as CD or DVD-ROM drives. ATA can also be called IDE, or integrated drive electronics. ATA comes in several speeds, the current ones being ATA-100 and ATA-133. The numbers 100 and 133 represent the speed of the interface in megahertz (MHz). Despite the increase in the speed of the interface, however, hard drive read/write speeds are limited by the rotation of the platters. In fact, the sustained speeds don't even come close to the limits of the interface, and any increases in speed are based more on marketing than improving the technology.

SCSI, which stands for small computer system interface, and is pronounced "skuzzy", is an interface used mostly in the server and workstation market due to the fast read speeds of these devices. Much like ATA, the interface is able to connect more than just hard drives, including printers and scanners. SCSI devices can even be daisy-chained, where one device is attached to another, which is then attached to another, and so on. There are a lot of different types of SCSI interfaces, and there is no compatibility between them. Although SCSI drives are often faster and more reliable than ATA drives, they tend to be much more expensive, and are therefore seldom seen on standard computers.

The successor to standard ATA, SATA stands for serial ATA, and has many people referring to standard ATA as PATA, or parallel ATA. SATA uses a much smaller physical connection type than ATA, and has a higher speed. Of course, because the speed is limited by the rotational speed of the drives, this increased bandwidth has little to no effect on overall speed. SATA does offer several advantages over regular ATA, though. With SATA-2, SATA drives are able to support native command queuing, or NCQ, in which read and write operations are prioritized in order to reduce the amount of time required for the platters to spin to the correct positions. It is also important to know that the SATA interface currently only supports hard disk drives, with very limited support of optical drives, which varies by manufacturer. Eventually, however, SATA will likely replace ATA as the interface of choice for all drives.