In special relativity, a light cone (or null cone) is the surface describing the temporal evolution of a flash of light in Minkowski spacetime. This can be visualized in 3-space if the two horizontal axes are chosen to be spatial dimensions, while the vertical axis is time.

Image from wiki: http://en.wikipedia.org/wiki/Light_cone

[img]http://upload.wikimedia.org/wikipedia/commons/thumb/1/16/World_line.svg/300px-World_line.svg.png[/img]

The following simulation was created to help you visualize the light cone (for observer moving with blue dot at the left X-Y panel). Events from three observers are drawed with blue, green and cyan traces.