Hard drives provide a computer with persistent storage. The data stored on disk will not be lost when power is removed from the computer. Much like memory, a hard drive stores information as bits. However on a hard drive the bits are stored as a small magnetic fields. These fields can persist without power.
A hard drive also differs from RAM because a hard drive has moving parts. A hard drive is composed of multiple disks, called platters, stacked on top of each other. These disks are divided into concentric circles called cylinders Each of platter can store data. To read and write data from a platter there is a head. This head is attached to an arm that positions it over a particular cylinder. In doing so, the head is able to read and write data to the cylinder as it passes the head.
There are many ways to measure the speed of a hard drive. One is called the seek time of the drive. To read a particular piece of data from disk the head must first be positioned over the correct cylinder, then the platter must rotate so that the part of the cylinder to be accessed is under the head. The length of time it takes to perform these operations is the seek time of the drive.
Once the head and platter is positioned correctly, the drive can start reading data. The speed at which it can send data to the computer is also a measure of speed. This is known as the transfer rate. When working with large amounts of data the transfer rate can be more important then the seek time.
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