How long wood takes to burn depends on how dense and moist each piece of wood is. In terms of chemical composition, most woods are very similar, but dense woods burn longer and are considered to be better for burning.
Wood for burning is broken up into two categories: hard and soft. Hardwood is denser and has higher energy content, which results in more heat being released per cord. A cord is how firewood is measured and equals 128 cubic feet. Hardwoods include ironwood, rock elm and oak. Softwood has less density, burns faster and produces less heat per cord, but it is actually more abundant in the coldest regions of the world. Examples of softwood are balsam, spruce and pine.