Healthy Aquilaria wood is pale, light and worth almost nothing. Agarwood is what the tree makes when a particular mould gets into a wound: the heartwood floods the damaged area with a dark, dense, aromatic resin, and over years or decades that infected section becomes the most valuable raw material in the perfume trade. Grade the resin high enough and it outprices gold by weight.
That origin explains the economics and most of the problems. Only a small proportion of wild trees are infected, there is no way to tell from outside, and finding the resin means felling the tree. Wild Aquilaria across South and Southeast Asia was cut to the point that the whole genus is now listed under CITES, and most legal supply comes from plantations where trees are deliberately inoculated, drilled and infected on a schedule.
The names are worth keeping separate because the traditions behind them are. Oud is the Arabic, and in the Gulf a chip burned on charcoal for a guest is ordinary hospitality rather than ceremony. Jinkoh is the Japanese, and there the material is not burned but heated on mica over ash so that it never smokes. Chen xiang, sinking incense, is the Chinese, and the name is a grading test: the best grades are so loaded with resin that they sink in water.
Sanskrit agaru sits behind all of them, and the trade routes that carried the word carried the wood.