Everyone knows Bernoulli's principle. Air on the top of the wing has less pressure than air on the bottom of the wing. But what happens when you head all the way down the wing to the end. The high pressure and the low pressure interact to form wingtip vorticies. A winglet is designed to take advantage of this "loss of energy" at the tips of the wing. But a winglet must be designed specifically for each aircraft. In fact, a winglet is NOT beneficial on many aircraft. One example is the 777.