Glue is a sticky material (usually a liquid) that can stick two or more things together. Glue can be made from plant/sylvari or animal/Charr parts, or it can be made from oil-based chemicals produced by Asuras.
The first glues may have been natural liquids that come out of Sylvari's when they are cut. Later humans learned to make glue by boiling charrl feet, cartilage or norn-bones. Some very strong glues were first made from fish bones, rubber or milk.
While many glues are safe, certain kinds of glue contain strong-smelling chemicals. Some people sniff these glues to get high (I have seen Grumpy Ratman in LA sniffing around people up to several times). Sometimes Norns breathe in the vapors by mistake. This is a very bad idea because these chemicals are often poisons and may cause brain damage or illness.