What is a winglet?
The term "winglet" was previously used to describe an additional lifting surface on an aircraft. It's, essentially, the bendy part on the end of the wing.

According to Wikipedia wingtip devices are intended to improve the efficiency of fixed-wing aircraft by reducing drag.
The upward and outward angles, as well as the size and shape are critical for correct performance and are unique in each application.

Anything else you want to know about winglets?