Metal (unless surface scratching with a diamond tip) and wood are similar, in that rotating carbide (usually half-round) cutters are used to cut a depth. Cutter selection and speed/feed rates are the difference between the substrates, including plastic. Metal does better with a lubrication; plastic and wood use suction to remove chips.
Glass is done with a rotating, faceted, diamond, and also lubrication. The best glass engraving is similar in quality to a sandcarved result and looks better than lasered glass (in my opinion), though there are many reasons lasering is preferred, such as being much faster.
—Gravotech
