It may be suprising, but wood as a fuel is carbon neutral. The carbon released by burning the wood is no more than if the tree we simply left to rot. Properly managed woodland in the UK supplies firewood for thousands of homes and ensures that for every tree felled at least one other tree is planted, to aborb the same amount of carbon as the burnt tree. On top of this, the careful manner in which trees are selected for felling means that each removed tree benefits its neighbours. There are also other wood based fuels; pellets or chips can be produced from wood waste, which is also used for kilns which dry wood in preparation for sale as firewood.
It is important to burn only suitably dry wood. Traditionally this means seasoning it but modern firewood producers use giant kilns fired by waste wood to dry the wood to the necessary 20% maximum moisture. It is possible to season your own wood, it should be protected from the rain and stored off of the ground. Cutting and splitting the wood will reduce the seasoning time as more water can escape from the exposed fibres. It will take between 12 and 24 months to properly season wood, depending on the variety of wood and the weather.
Seasoned wood will burn hotter than green wood and ensure both greater efficiency as well as fewer pollutants. Burning green wood will coat the inside of your chimney with creosote and tar. Seasoned wood will feel lighter, make a distinctly hollow noise when tapped and often start to split at the ends. Moisture meters are available to check that the moisture content is low enough.
Never burn processed woods (hardboard, MDF, etc) or treated wood (painted, stained, etc) as it can release toxic chemicals, not to mention make a mess of your stove.
Most modern wood burning stoves will have an efficiency rating of 80% or higher. This is acheived by both the inherant efficiency of wood as a fuel and technological advances,a mainly in controlling the flow of air within the stove and flue. This air flow ensures that the stove burns very hot and many stoves incorporate a 'secondary combustion' feature, subjecting the exhaust gases to great heat, burning up the particulate carbon before it can be released into the atmosphere.