Hard drives (or fixed disks) store information magnetically. They are actually made up of rigid cylinders and have a read/write head, much like any other magnetic recording device.
Because hard drives have extremely rigid, durable cylinders, they can spin at very high speeds. This means that the data on the disk can be accessed quickly (definitely faster than floppy drives and most removable media drives). Disk speed (or access time) is measured in milliseconds and is related to the time it takes a drive to access data. The lower the access time, the faster the drive. For example, an 11 ms drive would be faster than a 28ms drive.