We are all aware of many different forces. Tides, the wind, electric motors, volcanic and chemical explosions, or human muscles, can all exert force. However, physicists have found that all these different kinds of force that we know are really versions of four basic forces at work in the universe, each governing a different aspect of how particles of matter interact. Two of these basic forces (the strong and the weak mentioned below) operate entirely within atomic nuclei, and we are not normally aware of them. The other two (electromagnetism and gravity) account for almost all the forces we come across in our everyday lives.
How does force move from one object to another? It is easy to imagine an object exerting a force when it touches another, but there are many forces that are apparently exerted from a distance, e.g. the gravitational force between the planets and the sun, or the force that causes two magnetic poles to attract or repel each other. How does one particle or object exert force on another that is some distance away? In former times it was believed that force could be transmitted mysteriously through space. Today we believe that all the known forces are carried from one object to another by a special group of particles. Some of these, such as photons, are well known; while others, like gravitons, have yet to be discovered. In the discussion below we mention the carrier that we believe acts for each of the four forces. It is difficult to compare these basic forces because they all have different strengths at different distances. The relative strengths mentioned below apply only to particles fairly close to each other, as in the nucleus of a small atom, where all four forces can act simultaneously.
The most powerful of the forces, appropriately known as the strong or nuclear force, binds together the protons and neutrons (i.e. "nucleons") that constitute the nuclei of atoms. The force is carried by particles called mesons, which are constantly exchanged between the nucleons. This action apparently holds them together in the nuclear unit.
However, recent experiments have proved that the force carried by mesons is actually a secondary effect of the strong force inside the protons and neutrons themselves! During the past decade we have discovered that quarks are the most basic components of matter, and that they can join together in small groups to form protons and neutrons (which are stable within nuclei), as well as a large number of other short-lived particles. This new picture of nature's fundamental building blocks now leads to a new quest: physicists want to look at the basic strong force linking the quarks themselves within these small groups. Because of their main function, the particles that carry such a force have been aptly named gluons.
Next in strength comes the electromagnetic force, which has only 1/14 the power of the strong force at this distance, but its influence extends over a much greater span. This force acts between any particles carrying an electric charge. It keeps (negatively charged) electrons close to the (positive) nuclei of atoms, and so governs chemical reactions. Carriers called photons transmit this force between the particles involved. A stream of photons is what we call electromagnetic radiation, e.g. sunlight, microwaves, cosmic rays, X-rays, radio waves.
Next comes the weak force, with 1/10e8 the power of the strong force. It governs the radioactive decay of some elementary particles. Physicists have recently found the carriers that transmit this force, and have named them the W and Z particles. They are much more massive than protons or neutrons, and have extremely short lifetimes.
At first sight, gravity seems to be the feeblest force in the universe, with only 1/10e46 the power of the strong force. However, gravity maintains its effects over enormous distances and is therefore the dominant influence throughout the known cosmos. How is gravity transmitted? Scientists are looking for particles (called gravitons) as well as for gravitational waves. This is another area of the incredible, topsy-turvy world of subatomic and cosmic physics that awaits further exploration.
Current research tries to show that even these "basic" forces are really variations of a single universal force that appeared at the time of the original "Big Bang". This idea is called the Grand Unified Theory.