Gravity results from the warping or curving of space around massive objects, according to Einstein's Theory of General Relativity. All objects are affected by gravity, which pulls smaller objects towards the larger body.
The larger the body, the greater the gravitational effect. This means the larger an object is, the more it pulls other objects towards it, just as the sun's gravity keeps the planets in the solar system in orbit. On Earth, the spinning of the planet slightly lessens the effects of gravity. This means a person at the poles would theoretically feel the effects slightly more than if they were at the equator.
Isaac Newton's Law of Universal Gravitation states that any two objects in the universe attract each other by force, gravity, that is related to the mass of the objects and their distance from each other. The shorter the distance and the greater the mass of the objects, the more gravitational force they exert on each other.
As all objects in the universe exert force on one another, the relationships between the various celestial bodies are extremely complicated. This is why the moon is able to orbit the Earth and affect its tides, while the Earth is simultaneously orbiting the sun.