An object falls due to the force of gravity, as HowStuffWorks.com explains. Gravity is the force that pulls objects, including people, to the Earth's surface. Contributions from both Sir Isaac Newton and Albert Einstein inform current theories on gravity.
In addition to gravity, mass plays a role in the speed at which an object falls. Aristotle was the first to note that two objects fall at the same rate regardless of size because gravity is a stronger force than acceleration due to mass. The force of gravity occurs to different degrees on other planets and throughout the solar system. The moon orbits the Earth because of gravity.