An HDD, or hard disk drive, is a type of storage device for computers. They are sometimes called a hard disk, hard drive, or fixed drive.
HDDs consist of one or more magnetic platters sealed in an air-tight casing to protect it from dust. There is a magnetic head attached to an arm that moves across the rotating platter and writes data to it. The same magnetic head can also read data that was written to the disk.
HDDs are often inside a computer attached to the motherboard, or in an external case connected by a USB cable.
While HDDs are much slower than modern SSDs (solid state drives), they can have higher capacities, and are often cheaper. Because storage is cheaper for HDDs, they are often used to store data that isn't used often, or as long term backup.
HDDs are a non-volatile type of storage. This means that, unlike RAM, HDDs will retain any data that's written to them, even after you shut down the computer.