What are endplates? How do they work?
The air doesn't notice or care that we humans use so many different names for those wingtip extensions. They all help to keep the pressure difference up between both sides of the tip and add some friction drag.

Since giving basically the same thing such a variety of names is a pettifoggery and there is no standardization for naming them, it is hard to give a clear, distinguishing feature. If I now say that endplates extend to both sides of the wing and are smaller, while winglets point up only and are larger, I am sure that somebody will rightfully point out an exception to this rule. Just look at the latest monstrosity Boeing added to their new 737NG.



Boeing scimitar winglets (picture source)

When the term endplate is used, the transition between wing and extension is sudden and sharp, while winglets tend to have a smooth, rounded transition. Maybe this would be a better guide, but also here exceptions can be found.