They are made with different cultures (different types of good bacteria).
Yes, you can. My stepmother just recently started making it.
You have to buy what are called "kefir grains" (they come in a package with instructions.
I'm sure you can also find recipes online. Someone on Etsy sells kefir-making kits (using either milk or water and spices). I was thinking of getting the water kefir kit myself at some point.