Light is technically any form of radiant energy, but is almost always taken to mean visible radiant energy. This visible radiant energy is between 380 nm (a nm is one billionth of a meter) and 750 nm. Figure 1 shows how small a portion of the total spectrum the visible spectrum is. Light is a form of electromagnetic radiation, which means it has electric fields and magnetic fields vibrating back and forth very quickly as a wave. Because of the strangeness of quantum mechanics, light is also made of little particles with no mass, called photons.
Light, unsurprisingly, travels at the speed of light, which is the fastest anything can go. Light is bent (refracted) with various types of lenses for a number of purposes (in eyeglasses, in microscopes and telescopes, in projectors as in a movie theater, etc.). Light is a form of energy, which is why photovoltaic cells can harness the flow of sunlight to make electricity.
Light is needed for humans to maintain a high quality of life and is one of the major uses of energy (often electricity, but fire works as well, often in the form of burning biomass). Home lighting extends a person's day, and allows for many leisure and economic opportunities after the sun ceases to provide sunlight. People often get light from kerosene lanterns in developing countries and from a variety of light bulbs in developed countries (like LED light bulbs, CFL light bulbs, and incandescent light bulbs).
The study of light has an entire branch of physics devoted to it called optics; hyperphysics is an excellent place to learn more about optics.