Proudhon was an anarchist thinker. His main idea is that of free association of free individuals. These days his ideas are closely related to that of P2P. He also advocated the idea that no one can own more than what s/he can produce with its own hands.
He became famous for his saying "Property is Theft", which relates to the above idea. Most people own much more than what they actually produced by their own hands. They have gotten to own property because of power they have appropriated, gotten or received, and which is based on social-economic or social-political inequality.