A hard disk is a type of data storage device that stores data on a magnetically-charged spinning platter. It is a non-volatile storage medium that continues to store data even when disconnected from power. A hard drive may contain a single hard disk platter, or it may include several platters stacked on top of each other in a spindle.
Hard disk platters are made from glass, aluminum, or ceramic material and covered with a thin magnetic coating. This coating is made from a cobalt alloy that forms microscopic grains on the disk surface that can each hold a magnetic charge, representing binary data — either a 0 or 1. These grains are so densely packed that some hard disk platters can store a terabyte of data per square inch. Stacking several hard disk platters into a spindle allows a single hard drive to hold more than a dozen terabytes. A thin actuator arm sits along the top and bottom of each platter, detecting and changing the magnetic charges of each grain on the surface to read and write data to the disk.
Since hard disks involve moving parts, they cannot read or write data as fast as SSDs. They are more susceptible to damage. For example, if an actuator arm were to come in contact with one of the platters as it spins, it would scratch the surface and damage the disk.
NOTE: The terms "hard disk," "hard drive," and "hard disk drive" are often used interchangeably. "Hard disk" refers to the platters that store data, while "hard drive" and "hard disk drive" refer to the larger device that contains the disk platters, controller chips, and other circuitry required to interface with the computer's motherboard.