Plants grow and stay healthy through photosynthesis, a pretty amazing chemical process. As long as they've got chlorophyll (that stuff that makes them green), a light source, water and nutrition, they're sitting pretty. But indoors, there's not necessarily a lot of light — especially in the winter or in dark areas of the home.
Enter grow lights. These lamps mimic daylight so plants get more time in the sun (so to speak). Grow lights can be used for timing and forcing seedlings, commercial hydroponic crop production, growing plants in the winter in dark locales, and a whole lot more.