One or more aluminum, glass, or ceramic disk that is coated in a magnetic media and is located within a hard drive to permanently store all your computer's data. When the computer is turned on, these platters will begin to rotate at so many rotations per minute (RPM). This rate varies depending upon the model of hard drive you have; an example of how fast a hard drive may spin is 7200 RPM. As the disk platters are rotating, the read/write head accesses information on one of the platters. To help store and retrieve the data from the platter, data is stored in tracks, sectors, and cylinders on each platter.
Above is a graphic example of what the inside of a hard drive looks like and examples of a hard drive platter. As can be seen in the desktop computer hard drive this drive has three different platters.