A lot of what they do is, using a special resin, they perform ion exchange in the water. Removing calcium (and other) ions and replacing them with sodium ions. Hence why filtered water tastes salty.
What I don't get is the fact that supposedly we're supposed to eat more calcium because a lot of people don't get enough (strong bones and all that) and we generally eat too much sodium already and are supposed to cut down.
In which case I can understand the use of one for say the glasswasher we have at work where it stops watermarks on the glasses, but for drinking water? Why? What's the benefit of it?
I know in the USA they do something similar with their wine exchanging potassium for sodium to stop the formation of tartrate crystals, but it's illegal to import wine into the EU that's had done to it. Hence, it doesn't find it's way over here... in theory.