It started in a baker's shop in "Pudding Lane". Most buildings in London at this time were constructed of highly combustible materials, wood, straw, etc, and sparks which emanated from the baker's shop fell onto an adjacent building. Fanned by a strong wind, once the fire had taken hold it swiftly spread.
ca. 13000 houses and 87 churches were destroyed, amongst them St Paul's Cathedral. The site where the fire started is these days commemorated by a large monument, and a tube station called "Monument".