Every electronic, electro-optical or electromechanical device gives off some type of electromagnetic signals, whether or not the device was designed to be a transmitter. This is why the use of cellular phones is not permitted on airplanes or in many areas in hospitals -- their unintentional signals can interfere with equipment sensitive to picking up electromagnetic radiation (EMR). Since World War II, scientists have known that the EMR that "leaks" from devices can be intercepted and, using the proper equipment, reconstructed on a different device.
The EMR that is emitted by electric devices contains the information that the device is displaying or storing or transmitting. With equipment designed to intercept and reconstruct the data, it is possible to steal information from unsuspecting users by capturing the EMR signals. For example, in theory someone sitting in a van outside a person's house can read the EMR that is emanating from the user's laptop computer inside the house and reconstruct the information from the user's monitor on a different device. Different devices have different levels of susceptibility to Tempest radiation. A handheld calculator gives off a signal as much as a few feet away, and a computer's electromagnetic field can give off emissions up to half a mile away. The distance at which emanations can be monitored depends on whether or not there are conductive media such as power lines, water pipes or even metal cabinets in the area that will carry the signals further away from the original source.
The U.S. government originally began studying this phenomenon in order to prevent breaches in military security. The government was using the technology to their advantage during WWII and realized that they needed to protect themselves against others using the same tactics against them. The name Tempest, or Tempest radiation originated with the U.S. military in the 1960s as the name of the classified study of what was at the time called "compromising emanations." Today the phenomenon is more commonly referred to as van Eck phreaking, named after Wim van Eck, the Dutch computer scientist who brought it to general attention in 1985 when he published his paper "Electromagnetic Radiation from Video Display Units: An Eavesdropping Risk?," in which he demonstrated that the screen content of a video display unit could be reconstructed at a distance using low-cost home-built equipment - a TV set with its sync pulse generators replaced with manually controlled oscillators.
Van Eck phreaking is a major security concern in an age of increasing pervasive computing. High-security government agencies are protecting themselves by constructing safe rooms that through the use of metallic shielding block the EMR from emanating out of the room or by grounding the signals so that they cannot be intercepted. It is possible, though costly, for individual users to shield their home computer systems from EMR leakage. However, more and more manufacturers are creating products off-the-shelf that are safe from van Eck phreaking.
While the name Tempest was the code name for the military operations in the 1960s, at a later stage the word became an acronym for Telecommunications Electronics Material Protected from Emanating Spurious Transmissions and an abbreviation of Transient Electromagnetic Pulse Emanation Standard.
The following links provide further information on EMR emanation and van Eck phreaking:
Electromagnetic Radiation from Video Display Units: An Eavesdropping Risk?
Soft Tempest: Hidden Data Transmission Using Electromagnetic Emanations