Hard drives are generally characterized, classified, and categorized based on things like their storage capacity, data access speed, physical form factor, and by their interface.
The capacity of a hard drive is measured in bytes. The capacities of modern drives are in the gigabyte (billions of bytes) and terabyte (trillions of bytes) range and are likely to increase. Capacity is a factor of the number of platters or disks that are installed in the drive and the density of the magnetic storage capacity of those platters.
The hard drive is an electromechanical device. The data that is stored on the magnetic platters is read by a head that floats just above the surface as the disk rotates below it. The read-write head must move to different parts of the source as it spins to read all parts of a file. The combination of the speed of the head movement and how quickly the coil can rotate under the head form the basis of the access speed.
The first hard drives were huge, housed in separate machines and connected to the CPU via heavy cables. Modern hard drives are limited to three physical formats: 3.5-inch, 2.5-inch, and 1.8-inch. The smaller physical size limits the number of plates and the diameter of those plates. A 1.8-inch drive, for example, has a maximum capacity of 320 gigabytes.
The electronic connection between the hard drive and the processor has undergone several changes over time. Each interface change has improved the speed of data transfer and the ease with which the motherboard in the computer handles the hard drive. The current standard interface is SATA, the Serial Advanced Technology Attachment.