Another important point it's that two events (A & B ) are simultaneous only if a ray of light passing in A cuold reach B... and from here start all the paradoxes...
My english it's too poor to put here an extended explanation, but i can suggest you a great book about relativity: The ABC of relativity, by Bertrand Russel, who was a great friend of Einstein!
The only thing i want to say is that many people think that Einstein said that all it's relative, but that's not true: all it's relative about the observation point!
But it's all absolute using a calculating sistem based on tensores (which are cubic matrix used to change the reference sistem... too hard to explain for me
)
Anyway relativity seems to function even if isn't right...