First of all, there are two things you use water for. Misting and for the water bowl.

Misting: Here, it's just a matter of how clean you want the tank to be. Tap water is fine, but it'll leave quite a bit of mineral deposits on the glass. If you don't mind cleaning the glass of the tank, then there's no need for extremely pure water.

Water bowl: Here, you generally want water without some of the nasty stuff that's in our tap water. This usually mean that you either add a conditioner to the water, or just bottled water. Note that you do NOT want to use distilled water as the only water source, since it lacks even the good things in the water.

For a much more detailed description, I highly recommend reading this stickied topic:
http://www.frogforum.net/vivarium-te...hing-else.html