Milk kefir and water kefir are similar in that they're both drinks that use "grains" to produce a fermented beverage packed clear full beneficial bacteria and helpful yeasts. There is some overlap between the probiotic cultures each can contain, but there also some bacteria and yeast found in one that aren't found in the other.
Milk kefir primarily feeds on lactose, which is milk sugar found in the animal milk. Cow's milk is the most common type of milk used to make milk kefir, but it can be made from the milk pretty much any mammal it can be gathered from.
Milk kefir tends to be heavy and sour and is often used to make cultured dairy products and in smoothies-like drinks that are almost meals in a cup.
Water kefir grains feed on sugar water. They prefer sugars like sucrose, glucose and fructose to the sugar found in milk. The kefir made from water grains has a light, sugary flavor that can be used as a base for a large number of beverages.
Water kefir is water-based making it less filling. It's more often than not for hydration and can be consumed in large amount to rehydrate and quench thirst.
The kefir cheese is made using a yeast and bacterial culture called kefir, rather than an additional acid or rennet, to separate milk into curd and whey.
Kefir cheese is soft, creamy, and slightly tart. It is a versatile, simple cheese that can be used in all sorts of different ways.