A glyph is an object that is used for pixmaps, widgets, and
images of all sorts, as well as for things that “act” like pixmaps,
such as non-textual strings (annotations) displayed in a buffer or
in the margins. It is used in begin-glyphs and end-glyphs attached to
extents, marginal and textual annotations, overlay arrows
(overlay-arrow-* variables), toolbar buttons, mouse pointers,
frame icons, truncation and continuation markers, and the
like. Basically, any place there is an image or something that acts
like an image, there will be a glyph object representing it.