A hard disk is type of disk storage that stores and retrieves digital data from a planar magnetic surface. Data is stored or "written" by transmitting an electromagnetic flux through an antenna or "write head" that is very close to a magneticly polarizable material that changes its polarization due to the flux. Detection of the magnet fields on the disk, or "reading" is performed in a reverse manner, as the magnetic fields cause electrical change in the coil or "read head" that passes over it.
Usually several disks are attached to a spindle and along with motors and heads for reading and writing data the whole thing is encased in a sealed rigid metal case. The unit is sealed to prevent any contamination or dirt entering it which could cause a collision or "head crash" between the head and the disk and damage the disk surface of head.
Using a rigid disk and sealing the unit allows much tighter tolerances that in a floppy disk, consequently hard disks can store much more data than floppy disk and are much faster (at transferring data). In 2001, a typical hard disk might store 30 GB of data, rotate at 10000 revolutions per minute, and have a peak transfer rate of about 10 MB/s.