Hard Drives store the data on your computer.  In the beginning, hard drives only held a small amount of information, 120 MB or smaller.  Today, hard drive size has increased by leaps and bounds.  Computers come standard with 10 and 20 GB drives.  Hard drives are also getting faster.  The standard RPM for drives used to be 5400 and has now gone to 7200 and 10,000.  Hard drives can attach to the motherboard through an IDE or SCSI interface.  Hard drives can be internal or external, depending on your needs.  The hard drive pictured above is an internal drive.