The light that people can see is called visible light. In order to understand light, one must first understand what light is.
Light is actually electromagnetic radiation and is a very small region of the larger electromagnetic spectrum. The electromagnetic spectrum is a range of all the possible frequencies of electromagnetic radiation. These frequencies are classified into a number of different categories based on wavelength.
These categories (from shortest to longest wavelength) include gamma radiation, X-ray radiation, ultraviolet radiation, visible radiation (light), infrared radiation, terahertz radiation, microwave radiation, and radio waves.
Because of the way the human eye developed evolutionarily, we can only interpret a small portion of the spectrum as visible light. This portion is electromagnetic radiation with a wavelength between 400-700 nanometers. Interestingly, some other animals can see a broader range of the spectrum, and are able to detect ultraviolet and infrared as well.