Other than the wave particle theory. Seems like a simple question at first, but after some thought most of us will agree we don't know much. It comes out of the sun, lets us see things, and makes us warm. We can't grab it, can't outrun it, and can't seem to define it. Yet, physics has made many successful attempts at clarifying what light is and what it can do.
Light is just one portion of the various electromagnetic waves flying through space.
The electromagnetic spectrum covers an extremely broad range, from radio waves with wavelengths of a meter or more, down to x-rays with wavelengths of less than a billionth of a meter. Optical radiation lies between radio waves and x-rays on the spectrum, exhibiting a unique mix of ray, wave, and quantum properties.