A hard disk drive (HDD) is a type of storage device used in computers and other electronic devices to store data. It consists of one or more spinning disks coated with magnetic material, where data is written and read by a moving arm. HDDs are known for their large storage capacity and relatively low cost compared to other storage options. HDDs are commonly used for storing operating systems, applications, and personal files. They are slower than solid-state drives (SSDs) but can provide significant amounts of storage space, making them suitable for tasks like backing up data or storing large media files.