All wood is classed into three heat groups. The higher the group, the better the wood. What does the heat group tell you? It tells you the amount of heat, a specific wood may release when burning. Here are a few examples of what wood belongs to each group:
1st Group (2250 - 2500 kcal/kg) - Ash, Birch, Oak, Beech, Hornbeam, Maple, and Elm.
2nd Group (1800 kcal/kg) - Pine, Black Alder.
3rd Group (1500 - 1700kcal/kg) - Alder, Aspen, Lime-tree, Willow, Poplar and Spruce.
So now you have some knowledge about wood and can clearly see that it is not all that same when it comes to what you are using to heat your home with.