Listed below you will find the most important.
1.  PH
2.  Chlorine
3.  Hardness
4.  Alkalinity

PH - This is a measurement of the acid content of the water.  With a reading of 7.4 to 7.6 as being ideal, you can properly determine what you need to treat for.  Anything below 7.0 is acidic and can be harmful to your pool equipment and your pool plaster.  The same goes for a PH reading of over 7.8.

Chlorine - This is your disinfectant to sanitize your water.  You must maintain this disinfectant to prevent bacteria, algae and other organic matter in the pool water.  The four types of chlorine readings are FREE, DEMAND, COMBINED and TOTAL.
FREE - The total amount of chlorine available to attack new contaminants.  Should be a reading of 1.0 to 3.0 for pools.
DEMAND - The amount of chlorine that is needed to eliminate contaminants.
COMBINED - This is the amount of chlorine that has been used up and no longer is active.  An excessive amount of this combined chlorine can cause eye irritations and strong odors.
TOTAL - This is the sum of both free chlorine and combined chlorine residual.  (TOP)

HARDNESS - This refers to the total amount of calcium and magnesium in the water.  Too LOW will result in soft water which will slowly dissolve plaster and corrode metal fixtures.  Too HIGH will allow scale to form on plaster and equipment.  It will also give the appearance of cloudy water.  Ideal you should have a reading of 250 to 500 ppm. (Parts per million parts of water) (TOP)

ALKALINITY - This is the measurement of alkaline substances (bicarbonates and carbonates) that help buffer the water against sudden changes of PH.  Regular chlorine usage requires an alkalinity reading of 100 to 120 ppm.
(TOP)