It is not easy to give an adequate noise pollution definition. Basically noise pollution would be a form of pollution done by different audio sources that either distract, irritate or damage certain environment. Noise is really any unwanted sound, some sound that isn't natural to given environment and it causes disturbance to not only natural processes but can also cause harm to human society.
Noise pollution can become big environmental problem in some areas. It is not only stressful to many animals but can also cause the problems in predator/prey relation and detection, and even lead to different reproduction problems. More serious noise can even significantly reduce usable habitats for many sound-sensible animals, and can in the end even result in extinction of some species. For instance the noise pollution caused by man kills lot of sea animals, especially the ones that use the sense of hearing as the most important organ of perception like dolphins and whales. Scientists have found out that many whales and dolphins commit massive suicides by coming ashore guided by false signals that resulted from noise pollution.
Noise pollution doesn't have only negative effect on animals but can also have negative impact on human health. Exposure to much noise can lead to stress, aggression, sleep disturbance, hearing loss, and different other psychological issues. Many studies have shown negative effect of noise pollution to human health, and many people these days tend to find homes in some quiet and peaceful areas with where noise levels are very low.
The most common source of noise pollution is transportation, in form of motor vehicles. Some other sources like car alarms, emergency service sirens, office equipment, factory machinery, construction work, barking dogs, power tools, lighting hum, different audio systems, loudspeakers and noisy people can also cause noise pollution but motor vehicles are globally speaking dominant source of noise pollution.
Noise pollution problem definitely deserves more attention from environmental point of view. Though noise pollution is not widely recognized as one of the big environmental problems noise laws still do not exist in many parts of the world. If we look at the United States for example we can see that there are federal standards for highway and aircraft noise, many states and local governments have specific statutes on building codes, urban planning and roadway development but U.S. still doesn't have a single body that would regulate noise pollution. Not one country in the world does which means that effects of noise pollution are still underestimated.
In some countries different types of noise are managed by agencies responsible for the source of the noise, and so for instance transportation noise is often regulated by the relevant transportation ministry, while for instance loud music noise is a criminal offense in many areas. Basically world is still seeing noise pollution as the annoyance rather than the real problem, and this is the main reason why there aren't special bodies that would regulate noise pollution and suggest noise laws to legislative bodies.