There are four known forces in Modern Physics and they are gravity, electromagnetism, and strong and weak nuclear forces. The weakest of these forces is gravity which holds us on Earth. The strongest of these forces is strong nuclear which is the force that holds the atom and its parts together and it is the force involved in a nuclear bomb.
Einstein discovered the theory of relativity which explains how gravity and masses interact with each other with precise equations. It also shows how the fabric of space time moves with mass and gravity like in the picture below. He discovered that gravity actually travels at the speed of light and not instantaneously. The theory also explains how black holes gravity does not allow light to escape because the force is so strong.
Quantum Mechanics is the theory of the nature of atoms and subatomic particles. It differs from general relativity. It describes how things travel in both particles and waves. It is a world of probability where everything is chance. Space and time is very distorted in the quantum world.
Much of Einsteins life was devoted to the Unification of the Quantum world and gravity. He was looking for the theory of everything that connects the equations. He was looking for the master equation. He died before he could ever unify the forces of gravity and electromagnetism. String theory helps to further our understanding of both the small and large.
String Theory is the theory of everything that could unify the forces. It suggests that there are infinite parallel universes that we cant perceive or are outside of our observable universe. It says that there are ridiculously small strings billions of times smaller than atoms that make up everything in the universe. The theory also explains that there are 10 or 11 dimensions in which these strings can move but we do not yet understand. All of the mathematical anomalies have supposedly been solved in String Theory.