Fundamental force

There are four fundamental forces, sometimes called fundamental interactions. The forces are called "fundamental" because there is no simpler way for physicists to understand what the forces do or how they do it (their action). They are called interactions because the action of one object on another is matched by a reaction from the other. 
We feel the effects of gravity and electromagnetism all the time.
The strong and weak interactions are forces at the smallest distances and explain nuclear interactions.
A complete description of the forces requires advanced physics. The Standard Model explains 
three of these forces (electromagnetism, the weak force, and the strong force). Most physicists think that these become a single force under very high temperatures. This idea is known as the grand unification theory.
From the future to the past.
In the future, new forces may explain dark energy and dark matter. We may also get a Theory of everything that explains the four known fundamental forces. Even if we first "unify" the three forces of the Standard Model, we may continue to think of those forces as fundamental. This is what happened when theoretical physicists unified electromagnetism and the weak interaction with 
electroweak theory.