When modern warplanes have missiles fired at them, they deploy flares or chaff to lead those missiles off-target. The magnesium-containing flares are designed to burn hotter than the airplane's exhaust, drawing heat-seeking missiles to the flare rather than the plane. Meanwhile the reflectiveness of chaff—typically small pieces of aluminum or reflective plastic—are meant to dazzle and confuse radar-guided missiles. This overly dramatic video of a Eurofighter Typhoon shows you how it's supposed to work (at least with flares):

As you can see there it's dispensing the flares sideways. But there's another, more dense firing configuration they use that sends a third trail directly aft—the so-called "angel wing flare" pattern, whose name you'll better understand after you see the resultant smoke pattern:

With the way things are going in terms of global terrorism, I can't help but wonder if we'll one day see civilian aircraft equipped with ACDS.