In pre-Einstein mechanics, which goes back to the English physicist and mathematician Isaac Newton (1643-1727), gravity is a force with which masses act on each other. As other forces do, they cause bodies to accelerate.
In its simplest form, Newton’s law of gravity describes the force acting between two spherical, symmetric masses: The force with which the first sphere acts on the second is equal to the mass of the first sphere times the mass of the second sphere times Newton’s gravitational constant, divided by the square of the distance between the centre-points of the two spheres.
How to remove from this law more complicated gravitational effects, see the article The gravitation of gravitation. The differences between Newton’s gravitation and Einstein’s theory of gravitation, the theory of General Relativity, can be described systematically in the frame of the so called post-Newtonian approximation.