Light was the first creation of God, before and different from His creation of matter. It can refer to electromagnetic radiation of any wavelength, or to EM radiation of visible wavelengths  ; that stimulate the organs of sight, having for normal human vision wavelengths ranging from about 3900 to 7700 ångstroms (390 to 780 nm), violet has the shortest wavelength and red has the largest. It travels at the speed of light: about 186,300 miles (300,000 kilometres) per second.
EM radiation is a disturbance in an electric field, it consists of an energy particle called a "photon", a photon carries no electric charge and is massless, but since all energy is attracted by gravitation, light rays will bend near planets and stars.
Debate over whether light was a wave, or consists of particles was finally solved when quantum physics revealed all particles have wavelengths, but this is not generally noticeable in normal matter.
The EM spectrum consists of the following types of photon radiation (in order of increasing wavelength): gamma rays, x-rays, ultraviolet light, visible light, infrared light, microwaves and radio waves.
The Michelson-Morley experiment proved that the speed of light (and everything else that is massless) will always be measured by an observer to be the same, in every frame of reference, no matter how fast the source or the observer are moving, this gives rise to the theory of relativity.
As with any wave, wavelength times frequency equals the speed of a light wave.
In the Biblical creation God created light on the first day; God said Let there be light, and there was light. We must thus deduce than light can exist without a source of its own, unless one counts God as a possible source.
Creationism and starlight
For a more detailed treatment, see Starlight problem.
A common criticism of the young-Earth creationist viewpoint is that if the universe is only 6,000 years old, how would starlight from more than 6,000 light years away have reached Earth?
Young-Earth creationists have proposed a number of solutions to this problem, the current most popular explanations involving time dilation fields which resulted in billions of years time passing in outer space while only 6,000 years passed on Earth.