In a vacuum, all objects fall at the same speed regardless of weight. The force of gravity acts on everything equally, causing each object to accelerate downward at the same rate. However, air resistance can affect how an object falls, causing light objects with more surface area to fall slower.
In an atmosphere, simultaneous falling requires both objects to have a similar aerodynamic shape. For instance, a one-pound metal sphere and a one-pound sheet of metal fall at different rates, because the greater surface area of the metal sheet creates more wind resistance. Two spheres of different weights but similar size fall at the same speed, since the airflow over their surface produces the same effect on each object.
Galileo demonstrated this principle in his famous Leaning Tower of Pisa experiment in 1589. He dropped two balls of different masses from the top of the tower, and both reached the ground at the same time, proving his theory about the universal nature of free fall. It was also famously demonstrated by astronaut David Scott during the Apollo 15 mission to the moon in 1971, when he dropped a hammer and feather together and both fell at the same speed to the moon's surface.