Creosote is the build-up of particulate matter and carbon from the smoke released by burning wood. It can build up on the inside of your chimney and create a fire hazard. Once creosote becomes glazed it turns very hard and becomes difficult, if not impossible, to remove by brushing alone. If you have a fireplace, then creosote will build up over time, so you should take precautions to ensure you don't have a chimney fire.
Apply a commercially available creosote removal chemical to your firewood before you burn it. Once creosote has become glazed it's nearly impossible to remove by brushing alone. There are a number of products that will chemically modify the creosote and make it easy to brush off the inside of your chimney. Visit the link in the resources section below for a number of available products.
Wait for the period of time specified on the creosote removal product you are using to elapse. Most of these products require time to chemically change the creosote glazing into a powder that can be easily brushed away.
Brush the inside of your chimney with a chimney brush. Use a flashlight to look up the inside of your chimney to see if there is any creosote glaze left. If it can't be brushed free then repeat the chemical deglazing process before brushing again.
Use creosote removing chemical additives in your fire on a periodic basis to prevent the build-up of creosote in the future. Preventive maintenance of your chimney is much easier than removing glazed creosote and it will reduce the risk of a chimney fire.
If the creosote glazing in your fireplace is very thick, it's wise to hire a professional chimney sweep. They have access to chemicals that are much more powerful than those commercially available as well as valuable experience in dealing with glazed creosote.
Creosote is flammable and burns very hot. If your chimney catches on fire it can very easily spread to the rest of your house. Inspect your chimney at least once per year and before you start a fire if it's been a while since you used your fireplace.