An Emacs Plugin is platform dependent code, probably written in c, that is dynamically loaded as an OS “library” through a ForeignFunctionInterface to accomplish goals that are usually outside the scope of Emacs. Mostly this means buffers could contain the core functionality of a wide variety of applications.

Examples include:

From WishList

It would be really cool, if the emacs display engine would support the drawing of circles, lines, boxes. This would allow the creation of vector graphics within emacs. It would be enough to have some graphics only buffers for a start. With such an addition one could write a xfig mode. Well, I am only dreaming. – StefanReichoer