Term visual pollution encompasses different visually unattractive elements of a certain landscape. This type of pollution doesn't have to be necessarily connected to environmental damage, and is in fact more an aesthetic issue that can vary from one person to other.
What this really means is that visual pollution basically refers to some elements of the landscape which certain person is not comfortable looking at. This can be any man-made construction, like for instance some building that you just don't feel like it belongs here as it prevents your ability to fully enjoy the view.
Visual pollution, in this sense, is always the result of human activity, as something artificial created by humans which prevents us to fully enjoy in the nature's beauty.
In other sense, visual pollution could also accompany other forms of pollution such as for instance water pollution. Seeing dumped waste floating in the water is not only a clear sign of water pollution but also a form of visual pollution, and a sight that certainly isn't pleasant to look at. Littering is, in this sense, also a form of visual pollution.
Visual pollution can also accompany air pollution, for instance when fossil fuels fired power stations release harmful air pollutants into the atmosphere, creating ugly smokestacks and smog.
There is also a strong link between light pollution and visual pollution (especially in form of outdoor advertising via billboards during night).
In recent years many people have been complaining that wind turbines are becoming large visual polluters, and this issue has been subject of many topics as wind farms have become a common sight in many countries, all over the world.
As already said before, visual pollution has primarily aesthetic character meaning that objects like wind turbines don't necessarily have to be classified as visual polluters, as some may indeed find them to be eye-pleasing. Of course, waste dumping and littering are something completely else, and I really do not see how someone could say that polluted river or junkyard isn't a major source of visual pollution.