Since the early 20th century, physicists have recognized four fundamental forces, or interactions, that encompass all known phenomena in nature. Three out of four have been characterized rigorously and mathematically by the Standard Model, formulated in the early 1970s. The four forces are the strong nuclear force (also known as the color force), the weak nuclear force (mediates beta decay), the electromagnetic force, and gravity.
At very high energies, the weak nuclear and the electromagnetic force unite (start behaving interchangeably), while at still higher forces, it is believed that the strong force unites with the electroweak, and finally, the strong-electroweak force unites with gravity. It is believed that all four were united an instant after the Big Bang, in the earliest stages of the universe's formation.
The strong nuclear force holds together protons and neutrons in the atomic nucleus. More specifically, it is mediated by the exchange of gluons between quarks making up protons and neutrons. It is 100 times stronger than the electromagnetic force. When nuclei are smashed in nuclear reactions, energy from this force is released. Described by the theory of physics called quantum chromodynamics, it loses all its strength in distances much wider than the atomic nucleus.
The electromagnetic force is that with which people are most familiar, and it is responsible for all chemical reactions and the most recognizable physical properties, such as light. It is mediated by photons, which make up all electromagnetic radiation, from cosmic rays to visible light to extremely low frequency radio waves. Both heat and light are made up of photons.
Electromagnetic force interactions are determined by electric charge. The reason people don't fall through a chair while sitting on it is that the negative charge of the atomic electron shells making up the body are repelled by the negative charge of the electron shells making up the chair. Photon waves diminish in strength according to the square of the distance of their source.
The weak nuclear force is responsible for a relatively small range of fundamental interactions. It mediates beta decay, which is what happens when a neutron breaks down into a proton and an electron or positron. Mediated by W and Z bosons, it is about a hundred billion times weaker than the electromagnetic. It only operates over short distances.
Gravity is the weakest of all forces, but the most pervasive in the universe because it is generated by all bodies with mass. Gravity is 1036 times weaker than the electromagnetic force, which makes it hard to analyze mathematically. The particles thought to mediate gravity — gravitons — have not yet been detected. Gravity is also distinct from the other forces in that it has not yet been integrated with the others in a rigorous mathematical way. Physicists have been searching for a theory to unify gravity with the other forces for almost a century, with no luck so far.