The first entry under "Glue" on the dictionary page cited below notes:
"[Glue is] a hard, impure, protein gelatin, obtained by boiling skins, hoofs, and other animal substances in water, that when melted or diluted is a strong adhesive."
This type of glue (and properly the only type of adhesive that the label applies to) is still used today, primarily in woodworking—specifically, cabinetry.
What a lot of folks (especially in the United States) of when they say "glue" is a white, chemical-based adhesive marketed under the name "Elmer's Glue"™.
According to the company's website (link provided below):
"Elmer's Glues are chemical based. They are made or formulated from chemicals which are synthesized (created by Man). These chemicals were originally obtained or manufactured from petroleum, natural gas and other raw materials found in Nature."