I see people on boards saying stuff like "my alkalinity is 30." What exactly are these alkalinity tests measuring, and what are the units? Is it just a different way of measuring pH? That's what I thought at first, but then people were giving their alkalinity AND pH measurements, so unless they don't know what they're doing and the store clerks managed to sell them 2 pH tests (which could totally happen, lol), that's not it.

I understand the concept of alkalinity, as I've been through the standard freshman chem classes, where ALL they talk about is acids and bases... I've titrated more than a person should ever have to titrate. But I don't know what it means relative to aquarium water testing.

The alkalinity is the way of telling how much buffer capacity your water has, how stable your ph is gonna be. I have a test kit for KH, which is the alkalinity, that shows me alkalinity from 0 to ...... in increases of ten, like an alkalinity of 10, 20, 30, and so on. The higher your alkalinity is, the more stable is your ph.
At our old place the water had basically nonexisting alkalinity, and my ph was always dropping, which I had to combat with buffers, like crushed coral. Now, here the alkalinity is around 50, which is not that great either, but at least I dont have any more trastic ph drops and less coral to put into the filter.
I would say, anything above 80 or 100, and you can be assured you are not gonna have huge ph swings in your water...
Thats why its recommended to test for KH (alkalinity) when you have problems with constant dropping ph.
PH and KH test kits come in separate packages, and its possible that you will find 2 different ph kits, dry or liquid, in the stores. They have one available for high range ph, water that is above 7.8. The regular ph kit only measures up to 7.6...
Hope that isnt too confusing...

Ok... yeah I knew what KH was, I just didn't know it was also called alkalinity. Seems like a weird name for it, to me.