These before and after pictures of a factory built chimney flue. That ugly matierial you see in the before is creosote. Creosote is a byproduct of incomplete combustion of wood, typically most prevalent when first starting a fire and when burning unseasoned wood. Creosote is very flamible and where large deposits accumulate in a chimney, a chimney fire is very possible and extremely dangerous. The normal temperature in a chimney during a regular fire would be around 500 degrees. When a chimney fire occurs the temperature can go as high as 1800 degrees. When they occur, they can destroy the chimney and set the house on fire. Removing the hazardous creosote and inspecting the chimney for safety and integrity are the two main purposes of a chimney sweep.
The stuff you see in the photo is dryer vent lint. And in this case the dryer vent was completely stuffed. Dryer vent fires are more common events than one might think. But if you look at it not necessarily unexpected. When you dry your clothes, you are using flame and very hot air, in the case of a gas dryer. Dryer vents can get full of a very fine lint, which is extremely flammable, that combination can produce “dryer vent fires”.