This is air pollution.  It is easy to see, and sometimes is called smog in major urban areas.  It comes from a variety of sources, from vehicles to factories to the byproducts of other pollution on the land.  It conjures thoughts of suffocation.  It is easy to explain, and the results are highly visible and well documented.

This is water pollution.  Like land pollution, its affects are immediate and clearly visible.  It comes from production byproducts and carelessness.  It harms the ecosystems both within the water and those on land and depend on life within the water.

Water pollution is easy to demonstrate.  A common image through the decades has been of oil-soaked birds.  Show this image to anyone and they will understand the effects of water pollution.

This is land pollution.  Normally, it collects, whether unintentionally or deliberately.  It destroys natural landscapes, and can also act as a catalyst feeding air and water pollution.  There is no more obvious example of the need to clean up pollution than from the images we see of garbage and waste spread across wide spans of land.

This is light pollution.  It is the least-understood form of pollution.  It originates primarily from non-shielded lights wherever there is civilization.  Most lighting over the past century has been omnidirectional - not focused to its intended target.  This causes stray photons to collect in the atmosphere.  In aggregate, all these stray photons create an artificial daylight that extends hundreds of miles beyond their sources.

Unlike the other major pollution types, light pollution can be difficult to quantify, show, and explain.  After all, many people, without even realizing light pollution exists, automatically assume it is a normal part of our world.  But artificial daylight is bad for the environment - for wildlife, plants, and humans.  All living things on earth evolved with the expectation of a natural night darkness.  We are only beginning to understand and accept the consequences of this very real pollution type.