Hard disks contain aluminium or ceramic plates on a spindle. Each has a magnetic sensitive coating. Between these are read-write heads, about 0.07 mil above the surface. Data is 'written' to the disk by rearranging the particles of the magnetic coating to represent ones and zeroes.

Drives are factory sealed to keep out dirt, which could cause a head crash, easily wrecking the drive.

Drive performance is measured by speed and the time it takes for the heads to move into position (seek time). When buying a hard drive, capacity is also paramount. Buy a drive with more capacity than you think you need, because over time you'll probably be glad of it! Also consider the buffer or cache size. This is a memory chip built into the drive which stores data so that the processor doesn't have to wait for the drive to catch up. A larger cache means faster access times.

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http://computer.howstuffworks.com/hard-disk.htm