Gravity's effect on objects can be summarized in the following way:
The earth has gravitational attraction to every object within its vicinity such that the closer an object is to the earth, the stronger its gravitational force.
Every object in our universe emits a certain amount of energy which comes from spacetime itself. This energy, called gravitons, has a variety of different properties and behave differently depending on their circumstances but they do not interact with each other nor light particles. In particular, gravitons have no mass and are affected by gravity just like any other particle.
In the diagram, the black line is our universe. It contains three different spaces. The white lines represent different distances that can be travelled within the universe. The large white area in between two neighbouring white lines represents a space that you could travel between the two objects if you shifted your location by 2 units to the right and 1 unit upwards. In physics terms this means that it takes 2 units of motion for your position to jump from A to B and 1 unit of motion for your position to jump from B to C (to move up and down).
The lower right black sphere represents spacetime itself. Inside it is a collection of things called "events". The white lines represent time traveling from left to right.
If you are standing still and something moves towards you at speed v, the force of gravity on that object is :
F = mv2 / r ( 1 /r ) where m is the mass of the moving object This means that the larger an object is and the closer it is to another object, the greater its gravitational attraction. Similarly the farther away two objects are from each other, as measured by their distance keeps increasing over time. This means that if two objects are twice as far away from each other, the gravitational force between them is reduced to half. This relationship holds true for any two objects no matter how big or small they are.
If you were to throw a baseball, the ball will continuously accelerate up wards until it reaches a certain speed where it will then drop down again. The same is true of any object in our universe which has momentum and is not bound to the earths surface. If an object moves faster and faster it has more energy (because its moving further and further away from you). Because its moving further away from you (its actually moving in a straight line), it takes more distance / time for you to catch up with it. This means that it takes more and more energy for you to catch up with the object.
Its at this point that we need to introduce the concept of momentum and energy.
Each moving object has its own momentum and its own mass. This means that if the object is attached to the earth like a rock then its mass will be much smaller than it would be if it were loose in space. This means that even though the object moves faster and faster, its speed will actually remain constant as long as you can grab on to it.
If you wanted to increase your velocity then you could have two choices. First, you could have more energy (which means that its density gets lower since more energy is needed to overcome the force of gravity). Second, you could apply a larger amount of force (which uses up more energy in order to slow down). The result is that when you apply a force, its momentum will increase and it will become harder to slow down or stop.
If we were to plot the same diagram using momentum and energy instead, the physics would be identical. However, this time we would use the symbols (J) and (E) instead of (mv2 / r). If two objects are twice as far away from each other then there is twice as much momentum required in order to get them to move at half the speed. Therefore the mass of an object is not directly proportional to its velocity but depends upon how much momentum it has.
Going back to gravity, there are a variety of different forces that affect objects in the universe. These are:
Gravitational Force. Electromagnetic force, Strong Nuclear Force, Weak Nuclear Force.
For each of these forces, the energy is created from events in spacetime itself. Each particle has its own amount of energy and momentum associated with it (just like a baseball). When a particular force acts on an object that is producing energy inside spacetime, the object will move as if its being pulled in by gravity.