Agarwood is not a particular type of tree. It is a rare, fragrant, resin-rich wood that forms inside certain trees, mainly from the Aquilaria and Gyrinops families.
When these trees are injured or exposed to stress, they can respond by producing aromatic resin within parts of their wood. Over time, the resin accumulates and transforms sections of the normally pale wood into darker, denser, highly aromatic material.
Its fragrance can vary greatly depending on the tree, its growing region, how the resin formed, and how the material is processed.
You can think of agarwood as a special aromatic transformation that takes place inside the tree, rather than a separate kind of wood that grows on its own.