HDD is short for "hard disk drive." An HDD is a storage device used to store data. Unlike RAM, which requires electrical power to maintain its state, a hard disk drive stores data magnetically. Therefore, it retains its data when the power source is turned off or disconnected.
Most computers include either an HDD or an SSD that stores your files. For example, if your computer has an internal HDD, when you save a file, it is written to the hard disk drive. Additionally, the HDD stores pre-installed applications and other programs you add to your computer. Since modern HDDs have storage capacities of more than one terabyte, most users do not even come close to running out of disk space. However, if necessary, you can connect an external HDD (often called and external hard drive for additional storage or for backup purposes.
Important: The term "hard disk drive" may be used synonymously with the terms "hard drive" and "hard disk." Technically, an HDD and a hard drive refer to the same thing, while a hard disk refers to the actual magnetic platter inside the drive case.
Updated: November 9, 2012