Burning wood with high moisture content, over a period of time, produces creosote which builds up inside your chimney.
Creosote becomes a danger to your home when too much of it has built up inside the chimney. Many chimney fires have been caused by the build up of creosote because the owner of the property didn't realise they should have a check regularly.
You can have creosote removed from your chimney using an abrasion based cleaning method with will not cause any damage. In some cases a chemical cleaner can be used which changes the creosote to a more liquefied solution making it easer to remove. Liquid cleaners tend to be far messier as they are sprayed on and then run down the chimney to the hearth.
Cre-Away powder can be a better option. Cre-Away safely and effectively modifies creosote so that it can be easily removed when your chimney is swept. Cre-Away neutralises creosote making it less corrosive to metal components and less flammable. It also absorbs the oils and moisture contained in some forms of creosote. Then there is the combustion inhibitor which helps reduce the chance of chimney fires. Finally, a magnesium catalyst helps break down third degree creosote when the flue is heated during normal use. Effectively the catalyst in Cre-Away turns creosote into a char that can be more easily removed.
Remember to always work with a professional chimney sweep who will check your chimney before deciding the best and safest way to clean your chimney. They can also advise about how often your chimney should be swept.