Unlike most forms of pollution, light pollution isn't persistent: turn the lights off, and the dark sky comes back immediately. Like other pollution, though, it is a side effect of industrial civilization: it comes from sources such as domestic lighting, offices, factories, street lighting, and lit sporting venues.
Light pollution is most severe in the highly industrialised, densely populated areas of the United States and Europe, but even relatively small amounts of light can be important for sensitive applications - most major optical observatories have zones many kilometres in diameter severely restricting light emissions.
Light pollution can be reduced by shielding street lamps so that they light the street below and not the sky above, and by turning off unneeded outdoor lights: for example, only lighting football stadiums when there are people inside saves energy and helps keep the night sky dark.
(For more information, see the International Dark Sky Association: http://www.darksky.org/)