photon as the carrier of the electromagnetic force
We are used to thinking of light as the photon: the particle of sunshine, radio waves, and X-rays. Quantum electrodynamics adds a second, less obvious job for the very same particle. The photon is also the messenger that carries the electromagnetic force — the thing that makes magnets cling, makes static electricity crackle, and holds electrons in their orbits around the nucleus. Light and force turn out to be the same particle wearing two hats.
Here is how the two hats connect. When you switch on a flashlight, charges in the bulb shake loose photons that fly off and reach your eye — these are 'real' photons, the light you see. But charges can also exchange photons that never escape to be detected, living only for the brief instant between emission and absorption. These are virtual photons, and a steady stream of them, passed back and forth between charges, is exactly what we feel as the electric and magnetic forces. The photon being massless is why this force reaches across unlimited distance.
Calling a particle a 'force carrier' is the heart of how quantum field theory describes every fundamental force: a force is a particle being exchanged. The photon is the cleanest example. The W and Z bosons carry the weak force, the gluons carry the strong force, and each was understood by analogy to the photon. So the photon is not just one particle among many — it is the template for what a force is at the quantum level.
Hold two magnets near each other and feel them pull. In QED's picture, the invisible tug is countless virtual photons streaming between the magnetized iron's electrons. The same kind of particle that, as a real photon, would let you see the magnets is also the kind that lets you feel them.
Real photons let you see; virtual photons let you feel — both are the same particle.
Virtual photons are a bookkeeping device inside calculations, not little balls you could ever catch in a detector; only real photons are directly observed.