Four fundamental forces are known thus far:
1) Gravity is by far the weakest force, but is the force that has the greatest large-scale impact on the universe. Unlike the other forces, gravity works universally on all matter and energy, and is (so far as we know) universally attractive. Any matter or energy anywhere in the universe attracts all other matter and energy in the universe because of gravity. Gravitons in theory transfer this force.
2) Electromagnetism or electrostatic forces are the forces between charged particles, such as the force between two electrons, or the force between two current carrying wires. Photons transfer this force.
3) The weak nuclear force mediates certain types of nuclear particle decay. The weak force is transferred by W and Z bosons.
4) The strong nuclear force is the force holding together the protons and neutrons inside the atomic nucleus. The strong force is transferred by gluons and it acts on particles that carry "color charge", i.e. quarks and gluons.
See also: Physics