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Carl - the simplest way to explain it is to say that no matter what speed you are travelling at, light will always appear to travel at the exact same speed to you. But some interesting changes occur. Lets say you jumped in a spaceship and started orbiting the earth at 90% the speed of light (relative to your stationary speed standing still on earth, or relative to an observer on earth). You orbit for exattly 24 hours going by your clock on your spaceship, but the people on earth observing you have actually had more time pass, for every 24 hours your clock ticks over, earth clocks tick over 55 hours! Also, to people on earth your ship would appear to be 2.29 times shorter, and 2.29 times heavier.
So as you can see, time, mass, and distance are all relative to the observer. You would look out and think your ship is 100 metres long, weighs 10 tonnes, and 1 day has passed, but people on earth would observe 2.29 days passed, a 43 metre long ship, and a ship weighing 29.2 tonnes. Which readings are true then? Well it is all relative to the observer!
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