A winglet is a type of part used to provide stabilization for rockets and planes inside an atmosphere. It is smaller than a regular wing (hence the name) and is sometimes (but not necessarily) also a control surface.
Currently the following non-controllable stock winglets are available in KSP:
|AV-T1 Winglet||500||0.04||2 000||12||0.3||No|
|Tail Fin||600||0.13||2 000||12||0.61||Yes|