There are loads of different tree species which means each is very different in its wood characteristics. Wood is broken down into two main classifications, Hardwood and Softwood. Hardwoods are angiosperms. Angiosperm is a scientific name for plants that flower. All hardwood trees flower and encase their seeds in fruits (like apples and acorns). Hardwoods are also deciduous meaning they have a seasonal lifecycle. They lose their leaves in the fall and go dormant in the winter. Their leaves are mostly flat and broad. Because hardwoods grow slowly, they tend to be denser than softwoods and therefore better for producing heat when burning.
Softwoods are gymnosperms which means they are non-flowering. Their seeds are found in cones. Softwoods are typically coniferous trees, sometimes called evergreens. They have needle-like or scale-like leaves that do not shed during the winter. Because they grow quickly, they tend to be less dense. Colder climates, like Canada, are ideal for softwood growth.
There are some softwoods that are denser than some hardwoods. However, generally speaking, hardwoods are denser.
Usually, hardwoods are regarded as better firewood because they are dense, they burn longer and produce more heat than softwoods. Hardwood fires also burn cleaner. That is, they produce less smoke and spark which is great for in-home burning.