Electronic CounterMeasures (ECM)
ECM is a relatively new area of warfare, born from the Twentieth Century's fascination with electronics and the electromagnetic spectrum in weapons technology. It arises in part, however, from an ancient practice (going all the way back past Sun Tzu) called deception. To practice deception, in a military sense, is to take action to increase the chances your opponent (and sometimes your notional allies) will conclude you've done/are doing/are about to do something different. This is quite advantageous when it works. It can also lead to disaster when it fails. For more information, see military deception.
During World War I, electronic devices began to come into widespread use. Telegraph communication was the most common, followed by radio communication (although the latter was still in its infancy). Once it became clear how important these avenues of communications could be to the conduct of modern war, all players began to eagerly spend resources on discovering means to deny their use to the enemy. Telegraphs were really vulnerable (at the time) to physical interference only; cutting lines, destroying power, etc. Radio, however, could easily be denied by simply broadcasting a signal on the frequency in use by ones opponent, resulting in awful noise.
It was during World War II, however, that the modern use of ECM really took off. The advent and adoption of radar was probably a large driver in this escalation; like radio, once everyone figured out how important radar could be, everyone set out trying to deny it to the enemy. The favored method of this was to broadcast powerful radio beams back at the radar site on its own frequency and those immediately near it. This would have the effect of drowning out the relatively weak signal caused by an actual radar return, and the operator of the radar would be unable to see targets.
With this, the arms racing got off the ground. There are a number of physical constraints to jamming which favor the defender. First, it's easy (sometimes) to switch frequencies on your equipment, meaning the jammer has to figure out where you've gone to and then switch their jammers there. There is usually a brief lag, enough that the system will work for long enough to see what's going on. Second, the very act of jamming is a giveaway that something is going on. Third, (related to the first), jamming multiple frequencies is difficult; either the jammer must accept that he cannot broadcast continuously on all frequencies (frequency-hopping) and therefore offer the defender small 'windows' of clear air, or he must pay a price in power requirements (that increases faster than arithmetically) to simultaneously jam multiple frequencies at once. Finally, especially in modern times, practitioners of ECM are finding out that operating jammers is a risky bet, because since they're (by design) even stronger than the radars or systems that they are designed to affect, they're extremely easy to find and target. Entire systems (ARMs) and platforms or missions(Wild Weasel, Iron Hand) are devoted to killing these systems.
Back to World War II. The other type of ECM, aside from jamming radars or communications, is active defense. This is the utilization of electronic means (typically broadcast) to disrupt the functioning of enemy electronic systems. One of the first examples of this took place off the Allied invasion of Sicily. The German Air Force (Luftwaffe) had a new weapon designed to destroy enemy shipping. It was named the Fritz-X, and was the precursor and parent of the modern SLAM. Essentially, it was a small radio-controlled airplane with a television camera in it. The view from the weapon was relayed to a mother ship (typically a medium bomber) and an operator there would control the weapon using radio remote control.
The Allies were aware of the Fritz-X's existence. During the invasion and ensuing air attack by the Germans, the Allied naval vessels kept a close ear on the electromagnetic spectrum. When a Fritz-X was launched, they learned to identify the signals that went to it, providing early warning. They also learned to duplicate these signals at strengths aircraft transmitters couldn't possibly match, causing the missiles to go out of control and crash into the ocean. This is probably the first example of active ECM.
From there, things just got more complicated at an exponential pace. By the 1960s, the electromagnetic signature of a radar system (it's operating mode) became a closely guarded secret until its use. NATO and the Warsaw Pact both learned to send 'provocation' missions out designed to force the other side to turn on their targeting and search radars, and then carefully record the emissions of said radars so that they could be analyzed and countermeasures devised. The U.S. maintains to this day an enormously expensive, fairly secretive fleet of assets designed to do just that as well as to perform SIGINT based on such observations. See RC-135 Rivet Joint for an example.
In addition to jamming radars and radio, ECM systems have begun to reach out into the enemy aircraft themselves. The main radar in the U.S. AEGIS systems such as those aboard the DDG-51-class destroyer USS Cole are capable of putting out (at maximum power) approximately seven million watts of RF energy along an extremely narrow path. This level of emission is capable of literally frying the electronics of an aircraft or missile at close range (<1 km) or severely disrupting their function at longer ranges. Even in the air, the U.S. EA-6B Prowler jamming aircraft has the capability of disrupting the normal air traffic control systems of, say, the United States from Chicago to New York - from one airplane.
This, then, is ECM. There are of course answers to it - Electronic Counter-Counter Measures, or ECCM - but those swiftly begin to blend into the category of ECM itself. As with any other area of military technology, it's a competition, and the leaders tend to jealously guard their secrets.
ECM can be added to aircraft through the installation of a jamming pod such as the ALQ-131, designed to be hung from the ventral hardpoint of the F-16 Falcon. These can offer purely self-defense ECM capabilities to small planes. With enough power and pods, fighters can be made into limited-capability jammers; however, purpose-built aircraft are always more effective. The U.S. acknowledged this by continuing to remanufacture the EA-6B despite having retired its combat version, the A-6E Intruder several years ago. The U.S. Air Force even 'borrows' Prowlers to do its jamming…and if you know anything about interservice rivalry, that fact in itself is an astonishing admission of the Air Force's shortsightedness in retiring its own jamming platform, the EF-111 Raven.
So, onward and upward. Even now, if you were a seagull, you wouldn't want to be near the front of a U.S. jamming plane in combat. The EA-6B has a radiation symbol painted on the nose to warn ground crew and other aircraft not to approach the front (microwave antennae) when the jamming systems are in operation. Think microwave oven at a paltry 600 watts of RF energy…then think about the 7,000,000 coming from a SPY-1D…