Hard disk drives were a part of the first computers. The platters are magnetic and work with a head that moves back and forth, reading the data on the platters. The platters spin at variable rates, which have gotten faster as platter technology advanced. However, the speed of which these platters spin has been capped for several years. HDDs are traditionally cheaper as well, and they provide more storage space than an SSD.
The two most common form factors for modern HDDs are 3.5-inch, for desktop computers, and 2.5-inch, primarily for laptops. HDDs are connected to systems by standard interface cables such as PATA (Parallel ATA), SATA (Serial ATA), USB or SAS (Serial Attached SCSI) cables.