What the term "cloudy water" does not refer to: 
If you have just started your aquarium, or recently changed the gravel, and are experiencing a grayish brownish discoloration of the aquarium water, don't panic.
The gravel was probably still "dirty". The free-floating dirt particles should settle as well as get trapped in the external filtration fairly quickly.
A yellowish discoloration or brownish tint is typically attributed to organic matter. Activated carbon should adsorb this and eliminate this yellow tint. There is much discussion on how to use activated carbon to its fullest efficiency.
Some aquarists use carbon for only a few hours a week, some replace it on a regular basis - as they feel is best. A yellow discoloration is almost always an indicator of neglect too change the carbon - the carbon is simply used up.
What the term "cloudy water" does mean:
Green Water
The green water, often referred to as "cloudy water or pea soup", which makes your fish disappear right in front of your eyes, is an algae bloom. Free floating planktonic single celled algae growing at a rate that turns the water green.
The cause is always the same, too much light and excess nutrients (nitrates) often resulting from overfeeding. Excessive light cannot only be attributed to the aquarium lighting but also intense room lighting and direct sunlight.
Keeping nutrients low can prevent green water, while water changes provide little to no help in clearing the water. The algae spores are readily available in the water, including most waters used for changes. These spores will thrive in nutrient rich water, adding to the deterioration of the visual appeal. An algae bloom can become so severe that the content of your aquarium can literally vanish in the green water.
Some insist that turning off the lights will eliminate the problem, only to add that the problem just keeps re-occurring. So this hardly seems like a solution.
Algae will consume oxygen at night during photosynthesis. A severe algae bloom can deplete the tank of oxygen, so adequate oxygenation must be provided during the light off period.
Dying organic matter creates phosphates, so the filter should be rinsed more frequently during an algae bloom, eliminating some of the decaying matter. Vacuuming the gravel will also help.
Another symptom of excessive nutrients is white cloudiness.
White Cloudy Water
White cloudiness is a result of a bacteria bloom.
Sometimes the cleaning of all filters at once, or the changing of the gravel can trigger a bacteria bloom, due to the removal of bacterial colonies that had settled on the filter media or substrate. Another cause can be 
medical treatment of the tank using antibiotics, which may destroy these colonies.
The bacteria are either re-establishing themselves, or are feasting on high nutrients.
In nutrient rich water they can multiply at such a high rate that the water becomes cloudy white.
A bacteria bloom is cause for concern:
Bacteria need oxygen. A few grams of bacteria consume about the same amount as an adult human, again posing a threat of de-oxygenation in the aquarium. Immediate action is required if the problem is severe, or persists.
Once again the focus should be on nutrient control. Eliminating excess nutrients will starve the unneeded bacteria colonies resulting in a clearing of the water. A UV-Filter may be somewhat effective, as it kills bacteria. Nevertheless, the problem of high nutrients will not be fixed this way. Therefore the nutrients have to be lowered through water changes and the use of products such as Algone.
It is also advisable to check on ammonia during the period of a bacteria bloom, as ammonia may rise to dangerous levels.
In a salt-water aquarium, the protein skimmer usually helps to prevent and cure a bacteria bloom, as the bacteria are removed as "particles". Make sure the skimmer works properly when a bacteria bloom arises.
A severe bloom can create an oily film and enough foam-depressing agents to make a skimmer go flat (no foam no function). This has to be taken very seriously. Despite oxygen depletion ammonia gets more toxic at higher pH levels, which is characteristic for salt-water environments.
It is therefore vital to make the skimmer work again. An oxidation agent such as potassium permanganate can be used. Diluted and used sparingly (1% and only added in the low ml region) will oxidize enough fat that can be picked up by the skimmer, thus jump starting the system.
Nutrient Control
In any case, if you experience either green water or the white discoloration, you have a nutrient problem. It has to be addressed even if the cloudiness has disappeared.
