Particle physicists now believe they can describe the behaviour of all known subatomic particles within a single theoretical framework called the Standard Model. This model incorporates the quarks and leptons as well as their interactions through the strong, weak and electromagnetic forces. Gravity, alone, remains outside the Standard Model.
According to the Standard Model, the basic forces are transmitted between the quarks and leptons by a third family of particles. These are called gauge bosons, and they differ fundamentally from the quarks and leptons that are the building blocks of matter. There is a different type of particle for each force. Photons (the particles of light) carry the electromagnetic force; gluons carry the strong force; charged particles, and neutral particles, carry the weak force. A particle called the graviton - not yet observed - is believed to be responsible for gravity, but it has not yet proved possible to build a self-consistent theory that contains the graviton.
The fundamental forces appear to behave very differently in ordinary matter, but the Standard Model indicates that they are basically very similar when matter is in a high-energy environment. Theorists have found that the only consistent way to treat the weak force is to put it together with the electromagnetic force in a theory that describes a single 'electroweak' force. This discovery has been a tremendous breakthrough, just as the bringing together of electricity and magnetism was in the theory of electromagnetism, put forward by James Clark Maxwell in the mid 19th century.
