A hard disk (also called a hard drive) is a device that stores information inside a computer, kind of like a massive filing cabinet that remembers everything even when the power is off. Your computer uses its hard disk to save documents, photos, videos, games, and the operating system itself.

Inside a hard disk are spinning metal platters, similar to old vinyl records, coated with magnetic material. A tiny arm moves back and forth across these platters, reading and writing data at incredible speeds. The platters spin thousands of times per minute while the arm finds exactly the right spot to save or retrieve your files.

Hard disks can hold enormous amounts of information. A typical hard disk today might store a terabyte of data: that's roughly a thousand movies, a million photos, or enough text to fill several million books. When you save a school report or download a game, it goes onto the hard disk so you can access it later, even after shutting down your computer.

Modern computers sometimes use newer devices called solid-state drives instead, which work faster because they have no moving parts. But hard disks remain popular because they can store huge amounts of data for less money, making them useful for keeping large collections of files, backups, and archives.