The EMR that is emitted by 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, and more and more devices are being created that shield the EMR from leaking from the device. 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.

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 Complete, Unofficial TEMPEST Information Page  
The information on these pages is completely derived from publicly available, unclassified sources.

An Introduction to TEMPEST
A 2001 article from the SANS Institute.