There are four fundamental forces in the universe, sometimes called "interactions". These forces are gravity, electromagnetism, the weak force, and the strong force. They are called 'fundamental' because they cannot be reduced to anything simpler. Gravity and to some extent electromagnetism are experienced by people in their everyday life. A complete description of the forces requires advanced physics.
It is thought by most physicists that three of these forces (electromagnetism, the weak force, and the strong force) become a single force under very high temperatures. This idea is known as the grand unification theory.