Soot and creosote are by-products of burning wood and are formed out of the smoke going up the chimney. The form and amount of these deposits are dependant upon the temperature of the smoke. The cooler the smoke, the greater the buildup. These deposits accumulate on the inside of the venting system; i.e. stovepipes and chimney flues. They are flammable and, in sufficient quantities, will restrict the flow of flue gases, Obstructed flues can spill toxic flue gases (smoke) into the home. More information about creosote can be found here.