the xh's KH, GH, PH

definition
KH, or Carbonate Hardness, is a measure of the amount of carbonate and bicarbonate ions.

GH, or General Hardness, is a measure of the amount of calcium and magnesium cations.

KH and GH are typically expressed in parts per million (ppm) of calcium carbonate (CaCO3) or in degrees of hardness.

PH is a measure of acidity based on hydrogen.

Wonderful, what does that all mean?

in practice
GH
GH doesn't particularly do anything. It is the standard measure of how hard your aquarium water is. Some animals/plants like it harder, some like it softer. It really depends on the species that you place into your tank.

KH
KH is a buffer. Flex for us, KH. Corny jokes aside, KH contributes to your aquarium by making it more difficult for your PH to change drastically. Drastic PH changes can destroy livestock and damage plants very quickly.

PH
PH is kind of a complex topic as many things affect it and it contributes a lot to the health of your aquarium. But as far as PH goes, your livestock is going to prefer a certain PH, and the livestock will have some range or adaptability to allow it to survive in different PH water.

I haven't quite decided how to structure this information and further details, so I'll reupdate when I have a better idea. Bear with me please =).