What is a Hard Disk.

Hard Disk Drive Description
The sides of the hard drive have pre drilled, threaded holes for easy mounting in the 3.50 inch drive bay in the computer case. Mounting is also possible in a larger 5.250 inch drive bay with an adapter available at computer supply stores. The hard drive is mounted so the end with the links faces inside the computer.

The back end of the hard drive contains a porte for a cabel that connects to the motherboerd. The type of cable used will depend on the type of drive but is almost always included with a hard drive purchase. Also here is a connection for power from the power supply.

Most hard drives also have jumper settings on the back end that define how the motherboard is to recognize the drive when more than one is present. These settings vary from drive to drive so check with your hard drive manufacturer for details.

See also
http://pcsupport.about.com/od/componentprofiles/p/p_hdd.htm