"Winglets all work in the same basic way, but they don't all look the same. In essence, winglets reduce drag by recovering some of the energy in the wingtip vortex. This provides an effective increase in wing aspect-ratio (span² divided by area - a measure of slenderness), and therefore a reduction in lift-induced drag, for a smaller increase in span, weight and profile drag compared with simply making the wing longer
Once rare, winglets have become commonplace, and over the years their design has evolved to make them more efficient and maximize the fuel savings they provide. Here are some examples of how winglets have changed - and how they could change in the future."
"A third recent patent, also assigned to Airbus (above, US patent 8,387,922), uses the winglet for more than just reducing wing drag. Actively controlled surfaces are added to the trailing edges of the winglets to introduce aerodynamic instabilities into the eddies shed by the wing. These instabilities would accelerate the dissipation of high-velocity wake vortices and allow aircraft to follow each other more closely. (in the diagram above the cylindrical object (20) at the bottom is a structural housing for the active-surface actuators)."