Creosote is a combination of unburned gases and unburned tar-like liquids. When these come in contact with a cooler surface, they condense on that surface. In your chimney, it can appear as a black or brown coating. It can be sooty, dry and flaky, sticky, or hard and shiny. These deposits can reduce draft and cause smoke to spill into your room. Whatever form it takes, it is highly flammable. If large amounts build up, the result could be a chimney fire.