Theory of Gravity
Sir Isaac Newton in 1687 developed the Law of Gravitation, which stated that: Every particle of matter attracts every other particle of matter with a force proportional to the product of their masses and inversely proportional to the square of their distance apart. When written as an equation:
F=(G×M1×M2) / (d2)
Where F is the force of attraction due to gravity, G is a constant called as Universal Gravitational Constant, M1 and M2 are the masses of the two objects, and d is the distance between them.
Thus, according to this law you are pulling the Earth toward you and the Earth is pulling you toward it. As the mass of the Earth is much much larger than your mass, you are pulled strongly toward the Earth and this pull is called as gravity.
So gravity comes from mass and the more mass an object has, the more gravity it will exert.
Newtonian Law of Gravitation was considered absolute for nearly 200 years and remained undisputed, however Leverrier discovered a discrepancy in the orbit of planet Mercury which could not be explained even after considering the possibility of another unknown planet in vicinity. This particular discordance was explained by Einstein's General Theory of Relativity in 1916.