Heating efficiency of firewood is a product of how wood burns and the progresses through three stages of burning. In the first stage, wood is heated to the point where moisture is released. As the wood is losing moisture, its chemical composition is changing into volatile charcoal. The second stage is where moisture is mostly removed and actual flames burn off the volatile gases and liquids stored in the charcoal. Finally, the third stage occurs when the charcoal burns down and can be seen when coal embers glow. This is called "coaling".
This process can differ, depending on the species of tree, and very unique to conifers and hardwoods. What happens in these stages form the foundation of a species' heat value and determines the desirability of a wood as fuel. Read more in my article: Finding the Best Firewood for Home Heating