Agarwood forms as a resinous substance deep inside some kinds of trees from Southeast Asia. Many cultures prize agarwood, which isn't wood at all, as incense and perfume oil to use during religious ceremonies at temples and mosques. Excessive harvest of agarwood from supposedly protected forests has made the resin rare as well as endangered many species of host trees.
Also known as aloeswood, heartwood, or eaglewood, agarwood resembles amber resin. It is sticky and malleable, but not naturally produced by trees like most kinds of sap. It only forms within a small percentage of trees from the Aquilaria family, called thymelaeceae, that used to grow across the temperate and rainforests in Malaysia, Papua New Guinea, Indonesia, India, and Vietnam. These tropical trees actually grow very quickly in poor soil, so long as they have enough water.
Unfortunately, the trees aren't valued for their prolific lumber, but rather the anomalous substance of agarwood that seems to arise as a result of an infection or genetic mutation. Sadly, one cannot tell which trees might yield a hefty harvest of agarwood until they are felled and split open. Foresight may have allowed them to be monitored as a renewable resource, yet over-harvesting has all but eliminated the Aquilaria trees in most countries. Repopulation at this point is probably not tenable.
Agarwood, mostly from Vietnam, exported to other countries might find itself being burned as a medicinal smoke, wrapped with prayer shawls to scent them, or pressed to extract the potent oil. Ayurvedic and Traditional Chinese Medicine value the smoke as healing because it rebalances chi. In Korea the "kanam" gets burned for the black smoke, just like the "kanankoh" in Japan.
Holy places of Islam, Shintoism, and Buddhism use distilled agarwood oil as temple offerings and incense. The lauded smell from the "wood of the Gods" can be placed on altars as well as dotted on skin to bring out the rich scent. Even soaps and perfumes have incorporated agarwood's distinctive aroma.