What is Plato's concept of forms?
It's been a while since I studied this but from what I remember for my class on political philosophy at Cornell this is what I understand the theory to entail.
Plato uses the work "forms" to describe the true essence of material objects in the world. A chair is not defined by what we see but rather by its nature or "chair-ness." An object is therefore defined by its closeness to the idea or form of what it means to be a chair. Thus a tree stump, possessing the qualities of a elevated object conducive to humans sitting on it, has a quality of chair-ness and has the form of a chair. This idea of the form exists in a quasi-heavenly realm that can only be understood by the mind. The idea and application of the theory of forms is best illustrated in the allegory of the cave. In this story to prisoners are restrained and are looking at the wall of the cave. They are able to see the distorted shadows of things that are going by but not the actual thing itself. To Plato the material things we perceive are akin to the shadows while the thing producing the shadow is the true form. Plato believes that most people never break free from the prison of their own perceptions to grasp the reality of the forms. Those that do are the truly wise and learned that Plato would have us rise to the level of philosopher kings.