The classic example of gravitational force is that of an apple falling from a tree. Once its stem breaks, the apple accelerates toward the Earth's center of gravity at a rate of 9.8 m/s^2. The effect of gravity is observed between any two particles in the universe that have mass.
Gravity, unlike electromagnetism, is always attractive, and unlike the strong force, it works over long distances. This makes it the most easily observed force on the largest scales of the universe. The legendary insight of Isaac Newton was that this same force is responsible for holding the moon in orbit around Earth, as well as keeping the planets in their courses around the sun.