One part of the main concepts of modern physics is the General Theory of Relativity, a theory developed by Einstein during World War One; it gives an explanation on what exactly gravity might be:
Think of the universe as a giant rubber sheet, or a trampoline. This material, the "fabric of Space-Time", bends and warps whenever a mass rests on it - for example, a planet or a star. When another mass (such as a planet or asteroid) or a particle (such as one of light) passes by one of these indents in the Space-Time fabric, its own bath is warped and follows the path of the bend; orbits occur when one smaller mass rotates on the indent created by a larger mass - like a biker riding along the side of an emptied pool. The heavier the mass, the steeper the indent (and the faster the biker must go in order to prevent himself/herself from falling in). Sometimes, the sides are so steep that anything and everything, including light, falls in.
Quantum Mechanics, on the other hand, deals with forces and events on the extremely small end of the spectrum; it encompasses Electromagnetism, Strong and Weak Nuclear Forces, and the seemingly chaotic and unpredictable nature of atoms in their interaction with the known world - the opposite of the calm, predictable tendencies of General Relativity. Things are so random and unpredictable that you can never know the true outcome of an experiment until after it occurs - you can only make predictions.
One of Quantum Mechanics' forces is electromagnetism, a combined force of electricity and magnetism. This force is much more powerful than gravity, but doesn't have the same length of effect as gravity - only able to work within a range of a few centimeters or so. It is the same force that helps to keep atoms bonded together in molecules.
Another force is the Weak Nuclear Force. It is responsible for radioactive particles and energy, when the forces within an atom's nucleus is too weak to hold it together. This results in the decay of neutrons into protons, flinging out radiation in the process.
The last force is the Strong Nuclear Force. This is the force that keeps the protons and neutrons within an atom's nucleus together, preventing from the same-charged protons from repelling.