Alternatively referred to as disk space, disk capacity is the total amount of bytes that a disk drive or disc is capable of holding. Typically, the storage device is the computer's hard drive, but it can also be a USB thumb drive, a CD or DVD, a memory stick/card, or a floppy disk. Disk space is usually measured in kilobytes (KB), megabytes (MB), gigabytes (GB), or terabytes (TB). For example, a hard drive that is 500 GB is capable of holding 500 Gigabytes of information.
As shown above, when viewing the properties of a storage device, it shows Used space and Free space. Used space is what is already taken up by existing data on the computer and Free space is what is available to use.