What I want to learn from today’s discussion on Foucault's ideas

Foucault’s approach is broadly classified under the tag ‘New Historicism’. What is ‘new’ about the history that he does? He showed us how ideas like madness and sexuality which were taken to have no history at all (transcendental and metaphysical concepts so central to explain the ‘origin’ in historical interpretations) had a history and emerged at particular point in time in the history of Western civilization. Foucault seems to make a distinction between archaeology and genealogy as methods/approaches to history. What is exactly this distinction? In his essay “Nietzsche, Genealogy and History”, he charts out the details of a genealogical approach to history (he sometimes terms it as “Effective History”) against traditional approach to history. In his another essay “What is Enlightenment?” he says that his work is mainly “genealogical in design and archaeological in its method”. I want to know what is this difference that Foucault is pointing at? I feel that this difference is very central to the understanding of ‘what is it that Foucault did as a historian?’ Or to rephrase it, how did Foucault do what he did which resulted in shaking the traditional way of doing history?