Four Fundamental Forces
Gravity: Gravity is always an attractive force. When large masses put a dimple in the fabric of space, other objects orbit around it.
Strong Nuclear Force: This is the force that holds the nucleus together. It has an extremely short range. It holds the positive protons from separating.
Electromagnetic Force: This force binds molecules and atoms together using electric charge. It uses these charges to repel other atoms. That's why we don't sink through the earth.
Weak Force: This force deals with radioactive decay. During decay, protons replace some neutrons. It destabilizes atoms. The sun wouldn't exist without the weak force.
Einstein's General Relitivity
Einstein predicted that space and time was like a fabric or grid. Large masses bend that grid of space-time and cause a dimple that smaller masses rotate around. He also predicted that the speed of light was the speed limit on the universe. Light can also be warped by gravity as seen (or not seen) in black holes. The dimple is so large that light cannot escape.
Quantum mechanics is a mathematical theory that explains how extremely small particles and subatomic particles work and interact. It is thought of as "spooky" or strange because it has laws that we are not use to. For example uncertainty is an confusing part of quantum mechanics. By observing a particle you could change its probability of being a certain way. Things are not certain. Quantum mechanics deals with the strong and weak nuclear forces.
String theory is a work in progress that is trying to unify general relativity and quantum mechanics. It is our best idea so far. The idea is the world is mad up of vibrating strings of energy. These strings can be closes loops, two ended strands, or membrane like things. There are many theories within string theory such as M theory.