Experts generally do not actually know what gravity is, only what it does. Some scientists speculate that gravity is comprised of pieces known as "gravitrons" that move at the "speed of light." Astrophysicists sometimes say that gravity is a term to describe a force that attracts masses together or even sends them apart. Sir Issac Newton created the Three Laws of Motion that essentially convey how types of forces in nature relate to each other via concepts such as inertia and momentum. Newton's third law, "For every action there is an equal and opposite reaction," further defines how forces in nature cause particles to exert themselves on others.
Electromagnetism is a fundamental concept when describing the types of forces in nature. It is a theory (commonly known as "unified theory of electromagnetism") that describes the relationship between electricity and magnetism, such as how these occurrences may act on the other, with electricity generating a magnetic field, for example. Scientists use extremely complex formulas to define how these relationships work, but the basic idea simply talks about how charged particles (and their movement) change, manipulate or even create energy.
Strong and weak forces are two theories regarding the types of forces in nature that talk about how subatomic particles (and even smaller ones than those) essentially behave. A proton, for example, is generally comprised of smaller pieces such as quarks. The strong force describes the attraction holding pieces of a nucleus together, while the weak force is a term that describes the "transmutation of quarks," or conversion of very small matter in concepts such as "ElectroWeak Theory."
Electrostatic force is one of the reasons that molecules inside people actually stay together. In some respects, this force may even be stronger than gravitational forces that direct particles such as protons or electrons. Intramolecular forces describe attractions between atomic pieces of a molecule. Van Der Waals force is a term in physical chemistry to describe how tiny reactions drive these relationships via instances such as intramolecular bonding, for example.