Einstein's Theory of General Relativity runs on a model that can be demonstrated with a trampoline. a large object in the middle of a large plane creates an indent, causing surrounding objects to fall towards it. Space is much like this as large suns and black holes, by force of gravity, suck in planets their satellites as they circle around each other. He also found that space and time are interwoven in whats known as space-time. This plane of space-time is the trampoline in the model, and massive objects can warp it.
The Four Fundamental Forces are as follows: Gravity, Electromagnetism, Strong, and Weak nuclear forces. Each of these forces are stronger than the next with Gravity being the weakest and strong nuclear being the strongest. Gravity holds universes and clusters together among space, electromagnetism repels and attracts based on electron makeup, weak nuclear is radioactive decay and the loss of electrons which causes the atoms to become unstable, and strong nuclear is the force that holds protons and neutrons together in the nucleus of an atom.
Quantum Mechanics is a relatively new field of physics that is consists mainly of theories that we hope to one day solve, but some are thought to be impossible to test. The fundamentals of physics are 'thrown out the window' when it comes to quantum mechanics and potentials once thought as science fiction, such as teleportation and speeds faster than light.
The Grand Unified Theory is what many physicists strive to achieve; A theory in which Gravity and Strong/Weak nuclear forces can be explained and described mathematically using one equation. Thus far it has been impossible as the realms of Quantum mechanics and gravity have many unexplainable differences.
String Theory is a relatively new concept that theorizes that all matter is made up of incredibly small strands called strings. These strings vibrate at different frequencies, creating different things such as energy, light, forces, and matter. These strings however, may be so small that it could be impossible for humans to ever meddle or prove they exist.