The picture shows an example of a decay of a particle under the influence of weak interaction. In the process a neutron is transformed into a proton and an electron and a neutrino are emitted. The W- here acts as a force carrier. In the decay of the neutron, the down-quark is transformed into an up-quark and a W- particle. There after the W particle is transformed into an electron and an anti-electron neutrino.
Courtesy of The Particle Adventure.