Special Relativity comes up a lot here, and I've noticed that there is a lot of variation in some of the "explanations" of it I've seen here. Therefore, I thought I'd try to clear up some of the confusion with a step by step walk through some of the main concepts, concluding with how they all fit together to explain everybody's favorite punching bag: "The Twin Paradox".
Since some people's eyes glaze over when they see a bunch of equations, I'm going to try do it without the math.
1. Inertial frames of reference
Essentially, this is any frame of reference which is not accelerating. Imagine An astronaut(A) floating in space far away from any gravity source. His can be said to be at rest according to a inertial reference frame (We'll call this "A's frame"). If we add another astronaut(B) with a relative motion with respect to A, He is moving in A's frame, But is motionless according to another frame(B's frame).
So in A's frame, A is stationary and B is moving, and in B's frame A is moving and B is stationary.
We can choose from an infinite number of inertial reference frames, all of which A
and B would have different velocities, What frame of reference we use for any situation depends on which is more convenient.
One point about reference frames is that they are not the same thing as "point of view". A point of view is attached to an object or observer, a frame of reference is not. If A accelerates, his "point of view" goes with him, However, the frame of reference we originally called A's frame does not. Instead, after the acceleration, A is now motionless with respect to a different frame (A's new frame) and is in motion with respect to his old "A's frame".
(While accelerating, A is in what is known as a non-inertial reference frame. All you need to know about this right now is that that rules for dealing with non-inertial frames differ from those dealing with inertial frames.)
2. The postulates of Relativity
SR is built on two postulates:
1. The laws of physics is the same for all inertial reference frames.
2. The speed of light is a constant for all inertial frame.
The first simply means that there is no physical experiment that can distinguish between two reference frames. Any experiment you perform at rest with respect to one frame gives the same results as a those done at rest in a frame moving with respect to the first.
The result is that there is no "preferred" inertial reference frame. You can't say who is "really" moving and who is "really" at rest.
The second state that the speed of light in one of these physical laws that is the same for all inertial reference frames. And when we mean that the speed of light is "constant" we mean relative to the inertial frame from which it is being measured.
Example, going back to our astronauts A and B. Assume that their relative velocity is towards each other (A see's B approaching him and B see's A approaching him).
A flash of light occurs at a point of light between them.
The following animation shows how events occur in A's frame with the expanding circle representing the flash of light.