Yes, the kind sold in stores such as made by Helios is milk kefir. To my knowledge, no one has created a bottled water kefir yet.
Both water and milk kefir grains are a symbiotic culture of bacteria and yeast held in a polymer created by the bacteria. The difference is that milk kefir grains feed off the lactose in the milk while water kefir grains feed off some form of sugar (eg, fructose, sucrose, etc).
Milk kefir is a thick milk-based drink that's much like a slightly yeasty very sour runny yogurty drink. Water kefir is a mild to very fizzy tangy drink that's much like soda, except that while soda is full of chemicals, water kefir is full of probiotics, good yeasts, and beneficial acids created as a byproduct in fermentation, including lactic acid. The flavor of the water kefir depends on what you put into the ferment; water kefir is made with some form of sweetener, and some popular things to use in flavoring water kefir are lemon, ginger, fresh or dried fruit, and fruit juice.