The Four Fundamental Forces
Gravity:Gravity was discovered by Isaac Newton, and was the first force that was able to be scientifically understood. Newton is credited with coming up with the Law of Universal Gravitation which states that all objects are attracted to one another based off of the object's masses multiplied by the squared distance between them. Without Gravity we would float off into space on a tangent. Gravity works over a distance unlike how the Nuclear forces only work across distances on the subatomic level.
Electromagnetism: Magnetism and Electricity are both forces similar to gravity except that unlike how Gravity is only attractive Electromagnetism is attractive and repulsive. Like Gravity the two charged poles of a magnet are also inverse squared. The way that Electromagnetism works over distances is by Electric and Magnetic fields which can create one another. The fields have charges that reach out to other charges. If both charges are the same as in Positive Positive or Negative Negative then the objects will repel. But if the charges are different then the objects will attract.
Weak Nuclear Force: The Weak Nuclear force is stronger than Gravity, but it only works at very short distances. It is responsible for Radioactive Decay, which plays a key role in Nuclear fission. The Sun uses Nuclear Fusion to keep shinning, without it the sun would cease to exist.
Strong Nuclear Force: This force it the force that holds together Protons and Neutrons together to form the atomic Nuclei. This force contains a massive amount of energy, and when this energy is released like with the Atom Bomb it sets off a chain reaction that causes mass destruction. This process is called Mass Defect Energy. The range of attraction that this force has is about one femtometer or 1.0*10^-15 meters,
Einstein's Theory of General Relativity
Published in November of 1915 Einstein's Theory of General Relativity unified Newton's Law of Universal Gravitation with the geometry properties of Spacetime. This Theory suggests the existence of Black Holes which are regions of space where the Spacetime becomes so distorted that not even light can escape from them. This Theory also hints at the possibility of Wormholes, by stating that a big enough object could distort Spacetime. What this means is that the closer you are to the giant object the slower time appears to you, and that if you were to spend 15 minutes near the object then went back you your space station a much longer time could have passed then what you had perceived.
Quantum Mechanics is a branch on physics which describes the natures of small scale and low energies at the atomic and the subatomic level. It is also the mathematical description of the probable motion and interactions of Subatomic Particles. One cannot know the location of the subatomic particle, they can only know the probability that it will be there. It describes the workings of the Universe on the subatomic level, as well as the Fundamental forces excluding Gravity. The problem With Quantum Mechanics is that the equations for it and Gravity don't mix. You cannot look at the center of a Black Hole and use both equations to get that same answer.
Unification is a theory that binds Gravity with Quantum Mechanics thus binding the universe into one single equation. After his Theory of General Relativity Einstein worked for many year tirelessly trying to find a single equation that would describe the universe. Einstein worked on Unification even on his deathbed but was unable to come up with an equation that would link Gravity with Quantum Mechanics.
What is String Theory?
String Theory is a theoretical framework where the particles in particle physics are replaced with one dimensional strings. It is a Theory of Quantum Gravity. In String Theory the whole Universe is made up of these strings that Vibrate. These strings could help unify the Universe, but it is not testable.
There were originally five versions of String Theory which all required extra dimensions of Spacetime to make all of the math work out. These five Theories were united by Edward Witten into one big theory call M Theory. For this Theory to work there would need to be Eleven Dimensions for all the math to work out, with seven of the dimensions being unobservable. Witten discovered that the five String Theories of string theory where just five different ways of looking at String Theory. It is possible that Witten's M Theory could be the Theory of Everything.