Beginning algae growth caused by low or inadequate chlorine/bromine or other sanitizer/oxidizer levels.
Insufficient filtration caused by too small a filter (sized improperly, especially true with cartridge filters) or insufficient hours of filter operation.
Too frequent backwashing or cleaning of the filter (doesn't allow filter to "reset itself" or collect enough debris which actually aids in filtration).
Poor pool water circulation due to inadequate pump (horse power doesn't mean too much, sometimes smaller is better) or positioning of the return jets (always turn the jets down to the bottom of the pool).
Poor circulation due to lack of pool usage.
Overwhelming introduction of dirt & debris into the pool (overhanging trees, construction area nearby, etc.)
Heavy bather usage without proper additions of sanitizers/oxidizers.
Overwhelming introduction of "unfilterable" waste from swimmers (cosmetics, lotions, body fluids).
Use of Heater (water too warm) causing faster sanitizer usage.
Use of Solar Heating System (system is not being adequately chemically treated to kill build-up of biofilms, algae, etc.).
Here are some more...
Insufficient sunlight on the pool. UV rays are a GREAT natural oxidizer. (Indoor pools are a whole other issue that we will not address here.)
Lawn & chemical sprays "drifting" into the pool (from a neighbor).
HIGH pH causing sanitizer (chlorine) to work more slowly thereby allowing algae & bacteria to grow.
HIGH pH causing scaling to begin to occur.
HIGH Calcium Hardness causing scaling to occur. HIGH Calcium Hardness is due to regular usage of calcium-based chlorine (Calcium Hypochlorite such as hth, Sun, many others) or source water high in calcium hardness (over 300 ppm).
HIGH Calcium Hardness in conjunction with WARMER water temperature (over 85°) causing calcium to precipitate out (calcium dissolves & stays in solution BEST in cooler water) & cloud the pool.
Filter needing a chemical cleaning (remember that using a straight acid will only set the greases & oils into the filter media).
Pool not being vacuumed or brushed on a regular basis.
Trace heavy metals (iron, copper, manganese) present in the water.
Household cleaners and chemicals being used in or around the pool.
Broken or otherwise damaged filter lateral (sand), septum (DE) or element (cartridge).
Unclean solar blanket.  A biofilm is building on the side in contact with the water.
Make-up water added to pool from an unclean garden hose.
Dirty pool toys & floats putting a strain on the sanitizer.
Make up water that isn't treated the way it used to be.  Many municipalities are treating the drinking water with chloramines rather than chlorine;  they do an adequate job with bacteria but don't have the oxidizing strength to combat other non-lethal pathogens.  The switch was done to placate people's perceived "fear" of chlorine being in the water.
Lack of pool-owner care as in regular testing, vacuuming & maintenance.
When was the pool last shocked?  Hmm?

As you can see there are many reasons for cloudy pool water.  It's almost never caused by one item.  Normally it's a combination of thingss & you will need to go through this list (not exhaustive, if we're pushed we could probably think of a few more) item by item to see if it applies to your particular condition.

Prevention:
Prevention of cloudy water is preferred over treatment. Follow these steps to help prevent cloudy water:
1. Physically brush & clean ALL Pool surfaces weekly, including ladder steps (especially underneath each step) & rails
2. Expose ALL pool surfaces to as much sunlight as possible (sunlight & UV are natural oxidizers)
3. Regularly add oxidizing chemicals into the skimmer to purge & clean the filtration lines of any bio-film*
4. When adding make-up water from the garden hose, allow the water to run for 2 to 3 minutes before putting the hose into the pool to help flush out stuff that is in the hose.
5. Regularly test your pool water wither at home or at a local pool professional's store (just make sure that they're not using the same type of testing equipment you are) .
6. Regularly clean pool toys & floats
7. Regularly clean pool solar blanket  
8. Chemically clean pool filter every 4 to 6 weeks (use Filter Cleaner)
9. Add regular Maintenance dosages of "Shock" & Algicide every 1 to 2 weeks as prescribed
10. Use Soft Touch as a preventative measure to prevent algae growth.
11. Run the filter a minimum of 12 hours daily to prevent "dead spots" in the pool
12. Remember to clean & rinse the brushes, hoses & vacuums that you use to clean the pool.
13. Leave as much of your pool equipment exposed to the sun
14. Keep the water balanced at all times. Recheck after heavy usage or rain or large "top-offs" of new water. Water balance refers to Free Available Sanitizer level, pH, Total Alkalinity, Calcium Hardness.
15.  Pre-treat for metals with Stain & Scale (especially if you have well water).
16.  Shower before using the pool to help remove body oils, cosmetics, etc.
17.  Use a Skimmer Sock to trap pollen & other debris that would otherwise pass through the skimmer & filter system.
18.  Cut back overhanging tree branches that put in leaves, pollen, debris.
19.  Use the pool!

Treatment:
Treatment of algae MUST BE QUICK AND TOTAL! DON'T SKIMP!
1. Clean all pool & affected surfaces as prescribed above.
2. Physically clean & remove all visible algae
3. Add an initial dosage of algicide to the pool*
4. "Shock" the pool with a triple or quadruple dose*
5. Run filter 24 hours daily until water is clear & halogen or peroxide levels are maintained at a "higher" level*
6. Chemically clean the filter
7. Bring in a fresh water sample about 4 days later for a retest & evaluation
8. Maintain Soft Touch levels
9. Maintain good water balance of pH, Total Alkalinity & Calcium Hardness.
10. Use the pool!

*Follow the instructions given by your pool care specialist EXPLICITLY. Failure to do so could result in harm to you and/or your pool & its components.
NEVER DRAIN your pool.  The liner will shrink, voiding the warranty and could cause damage to your pool.  In gunite, plaster, or even fiberglass pools removal of the water from the pool could result in the pool structure "floating" and causing serious damage to the structure.

Information used by permission.