The strength of gravity depends on what two factors?

Newton's law of gravitation,which is a veryy accurate working model in most situation says that gravitational attraction between two bodies of mass M1 and M2,lying r distance apart is given by F=G*M1*M2/(r^2), where G is the universal gravitational constant

however,the correct theory of gravity is currently Einstein's general relativity which explains gravitation as curvature in space and time and according to Einstein's field equation this curvature (or gravity) is determined by the mass and energy density of an object.