Basically, gravity is dependent on mass, where all things – from stars, planets, and galaxies to light and sub-atomic particles – are attracted to one another. Depending on the size, mass and density of the object, the gravitational force it exerts varies. And when it comes to the planets of our solar system, which vary in size and mass, the strength of gravity on their surfaces varies considerably.
For example, Earth's gravity, as already noted, is equivalent to 9.80665 m/s2 (or 32.174 ft/s2). This means that an object, if held above the ground and let go, will accelerate towards the surface at a speed of about 9.8 meters for every second of free fall. This is the standard for measuring gravity on other planets, which is also expressed as a single g.