Relativity

Special relativity explains how to interpret motion between different inertial frames of references, which means 2 places that are moving at the same constant speed relative to each other in a straight line.  No acceleration and no curves.

Special relativity said the law of physics does not change in each of the frames.  The speed of light is constant to each observer in the 2 frames regardless of their motion relative to the light source.  For example, if A is traveling in one direction and A bounces a light to a mirror on the ceiling and subsequently catches the light by a detector on the floor.  If B is traveling in the opposite direction of A, he will see the light travel diagonally to the ceiling and diagonally again to the floor.  The path traveling by the light will be longer as seen by B.  As the speed of light is the same for A and B, the time must be longer to B (time dilation).

Special relativity only works in a special case - inertial frame of reference.  General relativity is to explain when the frames are not inertial.  Einstein called it the principle of equivalence which state that an accelerating system is physically equivalent to a system inside a gravitation field.  If you are on a plane and during take off, you fill a force pressing you into the chair.  This shows the effect of acceleration is "equivalent" to gravity.

For example, if one drop a ball in an accelerating spaceship, he will observe the ball falls as if it is in a field of gravitational acceleration.  The gravity pull is equivalent to the degree of acceleration.  If light enters a hole on an accelerating space ship and hit the opposite wall, the light will appear as bent as the ship moves on.  This is equivalent if a gravitational force is applied to the path of light (though the bend would have been very small).