General relativity is Einstein's theory that gravity is a result of warps and curves in the space of time. Ripples of gravity travel at the speed of light and are warped by the heaviness of planets. Space is a trampoline that heavy objects warp which is what we feel as gravity. A black hole is created when a nearly infinite density creates a concentration that warps space and allows nothing to escape it which also changes and bends time.
Quantum Mechanics is used to describe the micro parts of the universe at the atomic level. In quantum mechanics the world is seen as a game of chance. Anything could happen and it's impossible to know for certain the outcome to any experiment. Laws are very different at the quantum mechanic level than what we know to be true in our world. There is a even a chance that particles may pass through walls or other solid objects. With quantum mechanics each possibility may actually happen in many different universes parallel to our very own. The quantum forces include strong nuclear, weak nuclear, and electromagnetic.
Four Fundamental Forces
There are four fundamental forces. General Relativity is a very weak force that is still separate from the other three forces. The electromagnetic, strong nuclear and weak nuclear forces fall under quantum mechanics which deals with the micro parts of the universe. Electromagnetism creates a magnetic field. This force is stronger than gravity and an atom's negative electrical charge causes repulsion and that prevents us from falling through the floor. The strong nuclear force keeps protons and neutrons tightly bound in the nucleus and the weak nuclear force has to do with radioactive decay and gives off radiation.
The goal of unification is to describe everything in the universe using one set of equations or principles. The problem scientist have is that gravity and quantum mechanics don't seem to not be able to coexist. Math for the different sets of forces do not add up and a theory is dead when equations are inconsistent. Today the string theory is used to help unify small and large theories but is hard to test and still has work to be done.
String theories goal is to unify the understanding of everything in the universe to one set of principles and equations. In string theory tiny strings of energy vibrate in a multitude of ways creating everything in the entire universe. It is also believed that this theory requires around 6 different dimensions to work. The strings are thought to be 100 billion times smaller than an atom, but because of this it is almost impossible to test this theory.