Special relativity, developed by Albert Einstein, applies to situations where objects are moving very quickly, at speeds near the speed of light. Generally, you should account for relativistic effects when speeds are higher than 1/10th of the speed of light.

Relativity produces very surprising results. We have no experience dealing with objects traveling at such high speeds, so perhaps it shouldn't be too surprising that we get surprising results. These are a few of the things that happen at relativistic speeds:

• Moving clocks run slow.
• Lengths contract when traveling at high speeds.
• Two events that occur simultaneously for one observer are not simultaneous for another observer in a different frame of reference if the events take place in different locations.

That might sound very odd, but it only sounds odd because we don't see these effects in our daily lives.