The different matter particles interact with each other inside and outside the atoms.
This interaction is mediated by the force carriers:
*the photon* (electromagnetic interaction),
the three *vector bosons* W^{+},
W^{-} and Z^{0} (weak interaction) and
the eight *gluons* (strong interaction).
The fourth force in the universe is the gravity. It is much weaker than the others are,
even compared to the weak interaction, and it does not play any important role in
the world of particles.

The force carriers can be associated with two properties: strength and range. The strength is a measure of the size of the effect of the force, and of the probability of an interaction. The range is a measure of the typical area of effect of the force, and it is given in e.g. metres.