TEMPEST is a codename referring to investigations and studies of compromising emanations (CE). Compromising emanations are defined as unintentional intelligence-bearing signals which, if intercepted and analyzed, disclose the information transmitted, received, handled, or otherwise processed by any information-processing equipment.
The concept of TEMPEST is fairly simple. Basically every electronic device emits an electrical transmission while in use. These electrical transmissions are similar to radio waves in that you can build something that can receive and process these transmissions like how a car radio receives a radio station’s broadcast. Every electronic device due to the way the electronic components are laid out in the device emit unique signals. So if you can find a similar electornic device you could map out what signal a device emits for any action performed on that device. For example, say when you press the “W” key on your keyboard it sends a signal through your keyboard wire to your computer and this signal has a frequency of 1Hz. Then when you press the “Y” key on your keyboard the signal sent though the keyboard wire is 2Hz. Then someone in theory who already knows that the particular model of keyboard that you are using has this signal behavior could stand somewhere near your desk and determine what you are typing on your keyboard based on the electrical signals traveling through the air which are being emitted from the wire going from your keyboard to your computer. Now these electrical signals being given off by devices are usually VERY weak and a person would have to be standing very close to receive and decode them so you are relatively safe in this regard but it is something to consider if you are processing extremely sensitive information. Supposivly there are techniques to pull these electrical signals from power lines since in theory if your computer is plugged into the power outlet in your wall then the electrical signals from your computer are being leaked into the power line.
Here is a direct link to the NSA TEMPEST document: http://www.nsa.gov/public/pdf/tempest.pdf
A copy of the NSA document is also available on this web server at this link: nsa_tempest_a_signal_problem