Image: http://i.imgur.com/243EoyJ.jpg width=600
Also, for a little fun fact, those little serrated edges are called chevrons.
From my understanding, they eliminate so much sound that several hundred pounds of insulation can be removed from the fuselage. Link to chevron patent with more information
A gas turbine engine exhaust nozzle includes an exhaust duct for channeling a gas jet. A plurality of adjoining chevrons are disposed at an aft end of the duct to define an exhaust outlet. Each of the chevrons is triangular in configuration, with a base, apex, side trailing edges converging therebetween, and radially opposite first and second surfaces bounded thereby. The trailing edges of adjacent chevrons are spaced laterally apart to define respective diverging slots disposed in flow communication with the duct. The chevrons have a concave contour axially between the bases and apexes which promotes jet mixing through the slots.
The prior art includes various solutions for reducing jet noise and infrared signature. The solutions typically rely on vigorously mixing the hot core jet with the fan jet, or ambient air, or both, for reducing the velocities thereof and reducing the temperature thereof. In this way both noise and infrared signature may be reduced, but typically at the expense of engine efficiency and performance.