BIOMASS: is a general term for recently-harvested plant material to be used as fuel. As well as wood it includes bagasse (the fibrous residue from sugarcane or sorghum stalks); nut shells; pips and stones from olives and other fruit; straw; hay; dried seeds such as maize, barley or wheat.
PELLETS: Bio-fuel pellets made from wood (or, occasionally, grass) can be burned only on special-purpose stoves and automatic-feed boilers. They offer most of the environmental benefits of natural wood, with greatly improved convenience.
PEAT or TURF: Peat is woody material which has semi decomposed over about 1000 years. It is the earliest stage in the formation of coal. The nearly black moorland or bog peat should be dried and treated as for wood. Peat is highly volatile, easy to light (if thoroughly dry) and almost smoke-free. It will burn on open fires and closed stoves.
LIGNITE or BROWN COAL: Lignite is a natural mineral, intermediate between peat and coal, formed over 4000 years. It lights easily and burns well, though some varieties produce very large quantities of ash. Apart from a small deposit in Ireland it is rarely encountered in the British Isles, but is commonplace in Central and Eastern Europe.
COKE: Coke is natural coal which has been heated in a closed container so that the oily and smoky matter is driven off (the smoke is distilled to make, among other things, aspirin, creosote and ink). Hard metallurgical cokes (like 'Sunbrite') are extremely clean and long lasting, but rather bulky, can be very difficult to ignite and, unlike the softer Low Temperature Cokes will not burn on open fires.
PETROLEUM COKE: Sold as 'Petcoke', 'Longbeach' and under various proprietary names, is made from oil residues. Its exceptional heat, high acid content and lack of protective ash mean that grate and fire bar life will be drastically reduced, unless your fire has special chromium alloy bars. It is rarely sold unless mixed with another, less reactive, fuel.
HOUSECOAL: A general term for whatever form of coal is commonly supplied for domestic use. In the UK and Ireland it is bituminous coal, in North America it is anthracite.
SLACK: Dusty bituminous coal residues, 'nutty slack' if it contains significant lumps, while BREEZE has a similar meaning for coke.