Ionization simply means to gain or lose an electron. Water that has been ionized becomes either alkaline water or acidic water, meaning that the pH has been adjusted up or down. 

Alkaline water is full of Hydroxyl ions, and it is loaded with measurable antioxidants which make it great for drinking. 
Acidic water is full of Hydrogen ions. This water can be applied to the outside of our bodies to help soothe skin. It is also great for plant growth.