kefir, or what i used to think of as a yogurt drink when i was a kid, actually is a whole lot more and is by no means a watered down version of anything. kefir is like yoga for your internal systems - helping to balance both the digestive and immune systems. it's an enzyme-rich food so it helps to breakdown other stuff that you eat, plus it's a source of protein, minerals and vitamin b (an important one especially if you are a veggie).
simply put, kefir is a cultured milk drink loaded with that good for you bacteria. kefir is relatively inexpensive when compared to the hyped up probiotic yogurt drinks out there - try kefir, it does pretty much the same thing without the spendier pricetag at checkout.
you can make your own kefir, you just need some starter cultures or you can pick up some ready-to-go bottles in the dairy section of the grocery store.