QED is the theory of how light and matter interact. Electrons, positrons, photons — and the electromagnetic force between them, described to a precision that makes every other theory in physics look sloppy.
The magnetic moment of the electron, for instance. Theory predicts: 1.00115965246. Experiment measures: 1.00115965221. That's agreement to better than one part in a billion. If you measured the distance from New York to Los Angeles to that precision, you'd be accurate to within the width of a human hair.