The gravitational force (also referred to simply as gravity) is the force that pulls all the masses in the universe together. All masses attract other masses according to Newton's law of universal gravitation:
The gravitational force acts between all objects that have mass. It always attracts objects together, and although it is the weakest of the four fundamental forces, gravity has an infinite range. The force of gravity pulls us towards Earth, causing objects to fall. When objects have the ability to fall, that's called gravitational potential energy. Hydropower and tidal power are primary energy sources that take advantage of the gravitational force to generate useful work.
Unless someone is taking very precise measurements in a lab (see for example Cavendish's famous experiment which determined the gravitational constant, G), the force of gravity must include only really big objects (like the sun, the moon or a planet) to be noticeable. The PhET simulation below shows how small gravitational forces are between human sized objects.
The gravitational force is an example of an inverse square law, meaning that the force drops with the square of the distance. This means that if the distance is doubled the gravitational attraction is four times less. Differences in gravitational forces from the moon acting on one side of the Earth, the center, and the other side of the Earth lead to tidal forces.
At cosmological distances (much greater than the size of a galaxy) there is interesting current research on dark energy that shows that a previously unexpected repulsion happens with gravity at long distances, see here).
For more information on gravity please see here.
Below the Phet, there's a video from Scishow's series on fundamental forces, this one is on gravity.
The University of Colorado has graciously allowed us to use the following Phet simulation. To get a physical intuition about how the law of gravity works, please explore the simulation below. Notice that even the biggest gravitational force below is still quite small compared to how much a person weighs (about 500-1000 N).
The other videos look at the strong nuclear force, weak nuclear force, and electromagnetic force. Check out their youtube channel for more videos like these! (a wonderful resource for curious people).