To elaborate on what was said before, many of us like to see gravity as a special force existing between objects. That is to say, when we see a ball drop to the ground we assume that there is an invisible bond between the ball and the Earth which pulls them together. However, no one can actually detect this bond directly. We only believe it exists because we can see the results it causes.
Einstein began to change this perception of gravity when he introduced the principle of equivalence. To put it simply, this principle tells us that there in no difference between the forces of gravity and acceleration. This may sound quite strange at first but let us explain.
As you should know, acceleration is when an object picks up speed. Actually the true definition is a little broader, but this will do for now. When the speedometer on a car goes from 10 miles per hour to 30 miles per hour, it means that the car is accelerating. Humans can sense acceleration. When you are sitting in a car that is picking up speed, you can feel yourself being pushed back into your seat or, more appropriately, the seat being pushed forward into you. The faster the car is picking up speed, the higher its rate of acceleration, the harder this push will feel. For example, if you have ever flown in an airplane before, you will probably remember the incredible strength of the force which pushes you back into your seat when the craft rapidly accelerates for take off.
When you ride in an elevator, you can also feel the force of acceleration. Usually, when an elevator travels upward, it moves at a constant speed. However, to reach this constant speed from a stand still, it must accelerate. In the brief moment between when an elevator is at rest and when it is moving upward at a constant speed, passengers on board the elevator experience acceleration. To them it feels as if they are being pushed towards the floor. This push lasts only a second or two because after the elevator reaches its regular traveling speed it no longer continues to accelerate. However, if for some reason the elevator kept on picking up speed, the passengers would continue to feel the push of the elevator floor against their bodies. It would seem to them as if they had gained weight and in fact they would have. If you stand on a scale inside an upward accelerated elevator, you will actually weigh more than you normally do!
To make the point even clearer, let us take gravity out of the picture altogether. Pretend that you are in what appears to be a normal elevator. However, what you do not know is that this elevator is not on the Earth at all but is floating in deep space where there is no gravity to speak of. Now, pretend that there is a rocket underneath the floor of the elevator which is accelerating the elevator through space with you in it. Despite the absence of gravity, the force you feel as the accelerating elevator floor pushes against you is the same as the force you feel when gravity pushes you against the ground. If you let go of a ball inside the elevator, the floor will catch up to it with ever increasing speed so it will appear that the ball is falling. In short, it will seem to you as if there is gravity inside the space-born elevator and there is no test you can conduct that will prove otherwise.
The reason for this phenomenon, according to the principle of equivalence, is that acceleration equals gravity and gravity equals acceleration. They are one and the same. But, if they are the same, Einstein reasoned, why do we need both? Why must we believe in some invisible gravitational bond that no one can detect? Instead, he said that there is no special force of gravity, only acceleration. Yet if no invisible gravitational bond exists, then what is the cause of this gravitational acceleration? To answer this, Einstein wrote his general theory of relativity.
For more on Albert Einstein and the history of general relativity see History Page 7.
