You've probably already heard something about the speed of light. It's one of the most well known attributes of light. First of all, it is really fast. Light travels at 299,792,458 meters per second and is denoted in physics equations by a "c". (That's 186,000 miles per second for you non-metric people!) At that speed, you could travel around the earth more than 7 times. Ole Romer was the first to approximate the speed of light in 1676. He was kind of far off, but he had the right idea.
When Einstien developed his ideas of relativity, he stated that no object can accelerate to the speed of light or go beyond it. Essentially, light is a universal speed limit. Unfortunately, this means that science fiction spacecraft, like the Enterprise on Star Trek, couldn't really go warp 5.