Glue in Blizzard terms is the 'other' XML UI, all the UI parts that are not strictly or purposly made to be used when a user has entered the world sort of Glueing the whole user interface experiance together. More specifically the definitions of these UI elements are placed in their own folders in the WoW content files, and are used for things like the startup screen, login screens, and all of the other in-between parts outside of the game world. WoW reuses what WoWWiki terms as the FrameXML facility for other 2D UI elements, but calls it GlueXML rather than FrameXML in its folders to differenciate, as some kinds of elements are just altered copies for use outside the game world.