found a fairly understandable description of the differences.
h t t p://w w w.ehow.com/info_8053808_difference-between-deionized-distilled-water.html
(remove the spaces i added in the url to make the link work)

like edward said, its just the difference in the process and the equipment used to make 'pure water'.
but after reading that article actually ionized water is less pure than distilled water, at least that's what this article conveyed.

In either case the whole idea behind using 'distilled water' or 'ionozed water' is to use water that is not contaminated with the minerals that raw water usually contains. These minerals if used in a battery will bond to the plates and will also break loose during charging and discharging and settle to the bottom of the battery cells creating/adding to the sulfate buildup in the base of the cell that happens over time during charging and discharging.

An example of a sulfated battery is an older battery that when watering you look into the cell and see the light colored build up on the cell plates and the protective barrier on top of the cells, this crusty looking buildup is the minerals and sulfate left behind after repeated charging and discharging.
This build up will still occur even using distilled or ionized water but will be lessened considerably because the processed water has less minerals to create the contaminate that causes the buildup. I actually had an article that showed the differences and explained this in better detail but i can't seem to locate it at the moment (DOH!)

So even though it is recommended to use 'distilled or ionized water' , yes regular tap water or 'raw' water can be used but is not recommended.

Just FYI... to everyone... a simple google or bing search will find you just about anything you want to know about industrial battery maintenance info.