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Special relativity says that how things happen can look different to people in different places or moving at difference speeds--except for things involving the speed of light in a vacuum. Things moving at the speed of light always move at the speed of light compared to you, no matter how fast you're moving.
General relativity says that space and time are actually different aspects of the same thing--space-time--and that space-time is curved. Exactly how curved space-time is at any point in the universe depends on how much gravity there is in the area. In addition to bending space-time, gravity can also bend light, radio waves, and all kinds of other stuff.