The four fundamental forces are gravity, electromagnetism, weak nuclear force and strong nuclear force. For a long time these forces were split into general relativity and quantum mechanics.
Gravity, although described by Newton, Galileo, was later included into the ideas and concept of general relativity. It describes the force between two objects.
Electromagnetism is described as a force that appears between electrically charged particles. It is described as one of quantum mechanics' three forces
Strong nuclear force and weak nuclear force are the forces located at the subatomic particles. The strong nuclear force keeps the same charged particles from separating. The weak nuclear force causes radioactive decay.
General relativity was a concept that Albert Einstein came up with to describe gravitation and how physics applies to the planes of space. The concept is that space is like a piece of fabric that is pulled taught. A planet that is placed onto the fabric has a moderate amount of density and therefore pulls the fabric of space and time only slightly.
Black holes, like planets, also have a density and pulls the fabric taught. However, the density of a black hole is far more immense. This density resting on the fabric of space causes a deeper sag.
Quantum mechanics is the other sect of modern physics which deals with electromagnetism, strong nuclear force and weak nuclear force. However, certain parts of quantum mechanics and Einstein's concept of relativity disagreed and did not equate to the same answer. Physicists, realized that since they both exist within the same universe then they must have at least one unifying equation.
String theory, or the theory of everything, is the theory that everything is made of extremely small strings of energy. String theory was created by physicists as a means of unifying the equations of quantum mechanics and relativity via one equation. Scientists have been searching for proof of this theory but have determined that humanity simply does not have the means to test and prove or disprove the theory.
String theory also calls for ten or eleven dimensions. These include depth, width, height, and time, but also includes six or seven more dimensions so incredibly small that we don't have the technology to see more than half of them.