the gauge bosons
/ gayj BOH-zonz /
If matter particles are the players, the gauge bosons are the way the players talk to each other. Forces in the Standard Model are not invisible tugs across empty space; they happen because particles toss other particles back and forth. Those messenger particles — the ones that carry the forces — are the gauge bosons. Two magnets pulling together, the Sun shining, an atom holding its nucleus: each is really matter particles exchanging these carriers.
There are four kinds, one set per force the model describes. The photon carries electromagnetism (light itself is a stream of photons); it is massless and has unlimited range. The gluon carries the strong force that glues quarks together; there are eight of them and they tug so fiercely at short range that quarks can never escape. The W and Z bosons carry the weak force, the one behind certain kinds of radioactive decay; unusually, these carriers are heavy — about 85 to 100 times the mass of a proton — and that heaviness is exactly why the weak force has such a tiny reach. All gauge bosons are 'bosons', meaning they have a property called spin equal to 1 and, unlike matter particles, any number of them can pile into the same state.
The deep idea is that these carriers are not added by hand; they are demanded by symmetries built into the theory (the 'gauge principle'). Get the symmetry right and the existence and properties of the force carriers fall out almost for free. The Higgs boson, by contrast, is a different sort of particle and is usually counted separately rather than as a force carrier.
When two electrons repel, the Standard Model pictures them exchanging a photon — the same kind of particle that, in vast numbers, makes up a beam of light.
Forces are carried by exchanged particles, not action at a distance.
Gravity is not in this list: there is no confirmed 'graviton' in the Standard Model, and bringing gravity into the same framework as the other forces remains an open, unsolved problem.