What is light? To be truthful, it's a mystery. We don't really know what it is. But, throughout the years, we learned some things about it. Here are the very basics.
Light is a wave and it is a particle. Water waves are a good analogy to understand light waves. If you disturb water in a pond, you would see waves. But at the same time, water is made up of droplets. Similarly, light travels in waves, but in distinct packets of energy, called photons.
When you look at water waves, they go up and down, with peaks at equal intervals. That is called a wavelength. Light travels in waves which have peaks at equal intervals too. But light is made up of both electric and magnetic waves. Those waves can travel through air, water, or vacuum in space.
The wavelength determines what kind of light it is. There are radio waves, x-ray waves, infrared waves, microwaves, and other waves. The visible light we can see is only a portion of light. The amount of energy a packet carries depends inversely on its wavelength. So the shorter the wavelength, the more energy. That's why some light waves are relatively harmless to us, like visible light, infrared, and radio. And some are very harmful, like x-rays.
Waves of light travel at a speed of around 300,000 km/s. Hence, that speed is referred to as the speed of light.
Seeds, Michael A. Horizons: Exploring the Universe. 9th ed. Belmont, CA: Wadsworth Pub., 1998. 79. Print, p. 72
Image source: NASA