kefir is a fermented drink.. milk kefir is similar to a drinkable yogurt, its tangy like yogurt but not as thick... it is much easier to make than yogurt and can also be sweetened if you don't like the tangy taste... Yogurt is also a fermented food.
I make fruit shakes with milk kefir that are delicious.
Water kefir is a fermented water/fruit or water/juice beverage... I've just started brewing water kefir this past week.