Until the 19th century, it was believed that the two force: gravitational force and electromagnetic force, could explain all physical mechanism. The Gravitational force acts between two bodies with mass, and is given by Newton’s famous equation: F = k*G1*G2/D^2. Simply, it is the force that keeps two things together, keeps us to the ground, and also explains the movement and positions of stars and planets. However, the gravitation force was incapable to explain the binding of electron to the atom. The mass of atom is negligible, while electrons are fairly massless, therefore the force of gravity is almost zero between them. In order to explain this behavior, and also to explain the relation between all charged particles, scientists use the principles of Electromagnetism. As the name suggest, electromagnetism comprises of: Electrostatic and Magnetic force. The reason why charge flow from higher potential to lower potential, why a balloon clings to the woolen sweater, occurring of thunder and lightening and such phenomena can be explained by the laws of electrostatics. The law of magnetics describe how two magnets attract or repel each other depending on their polarity. The fundamental mechanism behind such attraction, however, has not been explained. The theoretical possibility of magnetic monopole, in fact, magnetism in overall, has been baffling scientists for over two centuries.
An English scientist by the name of Michael Faraday was the first person to understand the interrelation between electrostatics and magnetics and to come up with a basic definition of electromagnetism. Faraday experimented with a magnetic bar and a conductor wire, and noticed a change in current in his device whenever he moved the magnet away from the wire. Accounting on his experiment, Faraday postulated three basic laws on electromagnetism. Here’s a very short and well-explained video on electromagnetism: https://www.youtube.com/watch?feature=player_detailpage&v=cy6kba3A8vY#t=75s The orientation of the induced current, the point charge model, notion of electric and magnetic flux, diffusion current etc. was however still unclear. In 1865, James C. Maxwell unified all electromagnetic principles and published his ground breaking equations in his famous paper, “A Dynamical Theory Of The Electromagnetic Field”. From propagation of electromagnetic wave such as light, RF, microwave etc to the binding of electrons to the atom, and simply, the working of my laptop that uses electricity generated by rotating turbines filled with copper fans – Maxwell equations can explain everything related to electromagnetics.
According to Electromagnetism, like charges repel while unlike charges attract. This, however, contradicted with the structure of atom, as in atoms positively charged protons stay together inside the nucleus. To explain this phenomena, scientists discovered a force called the Strong force that kept the proton and neutron in the nucleus from busting apart. As the name suggests, the Strong force is so strong, it undermines the electrostatic force between the protons and holds them inside the nucleus. The Strong force is extremely complicated, and one must have the knowledge of fundamental particles such as hadrons, quarks, gluons etc to understand the process. Briefly explaining, Proton and Neutrons are called Hadrons and are composed of 3 Quarks. Proton is composed of 2 Up quarks (+ve) and 1 Down quark (-ve), while neutron is composed of 2 Down and 1 Up quark. Gluons are Strong-Force charge carriers that interact with the quarks and makes them change their Color. Gluons are extremely powerful particles, that prevent the quarks from straying away, thereby stabilizing the nucleus.
The Weak force, as the name implies, is the weakest of all fundamental forces and acts over a very tiny range. The weak force explains why some atoms undergo radioactive decay, the process that fuels every star in the universe, and indeed keeps the sun shining. Just like the strong force that changes the color of the quark, weak force changes the Flavor of the quark, by the impact of force carriers. The force carriers in weak force are called Bosons that interact with the quarks. Briefly explaining, Bosons are of two types – W boson that is either +ve or -ve, or Z boson that is electrically neutral. When a -ve W boson comes in contact with a Proton (you know what quarks Protons are made up of right? If not, see paragraph above), the proton changes to a neutron, as the -ve W boson changes the quark flavor of the proton from 2 Up and 1 Down to 1 Up and 2 Down quark. With change in the number of protons and neutrons, the atom as a whole changes i.e. decays to a more stable form.
I hope you understand the 4 fundamental forces of physics now, if not, at least the top two. Keep loving physics. It makes the world go round.