The theory of relativity explains force on an extremely large scale. This theory explains that gravity can only move as fast as the speed of light. Leaving us to believe the universe acts a sort of trampoline and the the more dense objects distort the fabric of the universe creating gravity. He also proved that gravity could not exceed the speed of light.
Quantum Mechanics describes forces on the atomic level. Dealing with Electromagnetism, Strong Nuclear Force and Weak Nuclear force. This theory explains the universe at an atomic level. Explaining particles motion and interaction within the atom. Showing that the universe may not be so predictable but more of a game of chance.
Unification is the quest for a theory that unifies all four forces into one theory that would explain the entire universe. The theories of Relativity and Quantum Mechanics directly contradict each other. Explaining the universe in different ways one that works on a large scale and another that explains the universe on a small scale. Einstein died still stumped by this idea of a theory that unified the universe in explanation.
String theory is an idea of physics that unifies all four of the fundamental forces and essentially could be the theory of everything. The idea that everything is made of extremely small vibrating strings. Theories suggest that the strings could be open or closed and that they're so small that no tests could be preformed to see them. These strings are thought to be 10 to the negative 33 centimeters. If this theory could be proven, it in fact, would explain the universe on both an extremely small scale as though quantum mechanics does and it would also explain the universe on a large scale, like the theory of relativity.