The total dissolved solids (TDS) are a measure of the quantity of minerals dissolved in the water. It does not give any information about the nature of the minerals.
When dissolved in water minerals will form individual ionic species which will act as carriers for electricity. Therefore a measure of the electrical conductivity of the water will give an approximate value of the TDS. Different standards may specify limits based on TDS or conductivity.
TDS have an effect on the taste of water, higher levels tending towards increasingly unpalatable water.
As a general guide, harder water which contains calcium and magnesium salts, will have a higher level of TDS and conductivity.
WHO limit = 1000 mg/l as TDS
EC GL = 400 uS/cm as conductivity