Electronic warfare, or EW, was any action involving the use of the electromagnetic spectrum or directed energy to control the spectrum, attack an enemy, or impede enemy assaults via the spectrum. Its purpose was to deny the opponent the advantage of, and ensure friendly unimpeded access to, the EM spectrum.
In Diaspora-era space warfare targeted communication, radar, or other systems of enemy ships and installations. Warships utilized a complex suite of electronic warfare systems. These systems utilized jammers, decoys, and stealth in order to help defeat or confuse incoming missiles. Modern warships had massive computer suites and emitters built into them that allowed them to confuse incoming fire and help conceal and stealth the ships' emissions.
Ships also utilized specialized drones and decoys that were deployed outside of the ship. The power requirements for these were so high that it was impossible to have them run on internal power; instead, they had to stay close to the ship on a tractor beam and receive beamed power. As a result, a warship could deploy only a handful of them at a time. (UHH)