Degaussing is a process of reducing the magnetism of a material by applying an opposite magnetic field. The name is derived from a unit of magnetic flux, the gauss. There are many different uses for degaussing.
Cathode Ray Tubes
Most televisions and computer monitors that use cathode ray tubes have a degaussing function. The electron beams cause a magnetic field to build up on the shadow mask, and this changes the path of the beams causing the picture to warp. An insulated bundle containing many turns of copper wire is wrapped around the front of the picture tube. When it is energized, the electromagnetic field minimizes the field on the shadow mask. One advantage of plasma and liquid crystal displays is that they do not require degaussing.
Degaussing is used to erase data on magnetic media, such as hard drives, floppy disks, cassette tapes, video tapes, and swipe cards. Data is stored by changing the magnetic properties of tiny areas of the media surface. When a strong magnetic field is applied, all data is lost as all the areas become the same. Degaussing is not required for non-magnetic media, such as USB drives which use solid-state memory chips.
Degaussing is also used in retail security tags. Rather than waste time removing the tags at the checkout, they are are passed over a degaussing device. This cancels their magnetic field so that they can pass through the door sensors without setting off the alarm.
Ships and Submarines
The huge amount of ferrous metal in large ships causes many problems. The magnetic field created by all this metal can be strong enough to make a compass deviate from magnetic north. Submarines can been be detected by a cable placed along the seabed, so when the submarine passes overhead it's magnetic field induces a current in the cable which triggers an alarm. There are even mines that are triggered by a ship's magnetic field, so that direct contact with them is not required for them to explode. This magnetism can be minimized by using permanent magnets or electromagnets to create an opposing field.