Double glazing has two panes of glass separated by a spacer with a gap (often filled with an inert gas like argon) and a Low-E (low emissivity) coating on one surface.
Triple glazing has three panes of glass and two gaps (so two insulating cavities), and often two low-E coatings.
Because of that extra pane (and extra gap), triple glazing offers higher insulation and better noise reduction—but at higher cost and added weight.
