Okay, you asked for it ...
GH - General, or permanent hardness, is called GH, and is the measurement of calcium and magnesium salts dissolved in the water.
KH - Carbonate, bicarbonate, or temporary hardness is called KH, and is the measurement of calcium and magnesium bicarbonates in the water.
pH - The term pH is a combination of the mathematical symbol p, representing negative logarithms, and H, the chemical symbol for hydrogen.
Basically, pH is a simple way of representing the concentration of hydrogen ions in the water. Hydrogen concentrations are extremely small numbers, so pH converts them to easy-to-understand numbers. Hydrogen ions are positively charged hydrogen atoms whose number in the water indicates how readily they will react with other substances.
The pH scale is a "negative log 10" scale, and goes from 0.0 to 14.0. Water with a pH under 7.0 is called acidic, and water with a pH over 7.0 is called alkaline, or basic. A pH of 7.0 is called neutral. Acidic water has many more hydrogen ions than alkaline water.
This is only part of a much more complete article that I wrote for another website. Feel free to use as is, paraphrase, ignore, or ask me for more. (Ha!) Seriously, for this site, I'd just go with the second sentence (highlighted) of the definition for pH.
Deborah