Basically, agarwood, sometimes also known as aloeswood, eaglewood, is a fragrant dark resinous wood formed in the heartwood of Aquilaria trees. When the Aquilaria trees become infected with a type of mold through the damages from storm or other forces, the infection will progress and the tree produces dark aromatic resins, that is Agar. It is like a scar on human skin but on Aquilaria trees
Different formation of agarwood will produce different aroma. There are two common ways: Red soil, where Aquilaria trees damaged, infected and aging under the red soil; Wild insect, where the damaged Aquilaria trees' wooden parts eaten by wild insect and leave the agar continue to age.
Agarwood has very high value on medication, but it's often used in incense, perfume, and small carvings. First-grade agarwood is one of the most expensive natural raw materials in the world due to its rarity and long aging progress. However, due to its high popularity, lots of Asian countries had started human plantations recent years to avoid further damages to wild resource in Asian tropical zones.