Gravity

Gravity is a force that acts on a system of bodies that is proportional to their masses but the force grows weaker as the square of the distance between the center of the system of bodies increases.  It is a physical force.  Yes.  It exists.  But we truly don't know why it exists.  There are pros and cons to gravity.  Like if one were to jump up in the air it would be bad if he/she would keep going and eventually leave the atmosphere and suffer consequences.  But at the same time if you falling and did not have any way to slow yourself down that could be a problem also.  Gravity is one of the four fundamental forces.  It plays a part in Einstein's general theory of relativity.  For a body that produces x amount of gravity it causes a dent is the space-time continuum. Also Newton described it as Newton's law of universal gravitation which it states that every point mass in the universe attracts every other point with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.  The force of gravity is very unique and is different at different parts of the universe.  For the force of gravity determined by the mass and distance.  Here on earth the force of gravity is 9.81 m/s^2.  That is how fast you will accelerate if you were to drop towards the center of earth if dropped from a given height.  If you wanted to calculated the force of gravity that the earth is producing on you would multiply the earth's mass, your mass, the gravitational constant k, then divide the distance between the two of you.

http://en.wikipedia.org/wiki/Gravitation
http://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation