Fundamental forces
Interactions can be classified into one of the following groups
Gravitational force
Electromagnetic forces
Weak interaction (responsible for beta decay)
QCD (between colored objects, mediated by gluons)
Strong force, holds nucleons together (not a true "fundamental" force since it is mediated by mesons which are quark-antiquark composites)
Many texts would list four forces, considering QCD part of the strong interaction.
In 1967 Weinberg and Salaam independently developed a theory where the electromagnetic and weak interactions are integral parts of the same theory. In this model, the W and Z bosons appear identical to the photons. However, through a process called spontaneous symmetry breaking ( which is mathematically similar to the symmetry breaking in a magnetic material where all the spins decide to pick a direction at random and orient their spins along that axis ) the W and Z bosons obtain large masses while the photon remains massless. This theory made predictions which were verified experimentally, and has forms the basis of the standard model. The other fundamental forces, QCD and gravity, remain elusive regarding attempts to include them in a grand unified theory (GUT) with the electro-weak theory.