It tastes like fizzy drinkable yogurt with a hint of yeast. You can drink it plain, but most people prefer it mixed with fruit and other additions into a smoothie. I love the taste. It's a great way to get probiotics and good yeast strains into your system. You can buy it from a store, but the homemade kind has more strains of good bacteria and yeasts. To make the real stuff at home, you need to acquire milk kefir grains.
saratc hit the nail on the head! I would definitely suggest getting your own grains if you're wanting to do kefir. I shared some with a friend of mine (who had the store bought ones) and she called me twice to ask if she was doing it right b/c she didn't think it could be that easy! Definitely easy!
To start you could always buy some at the store and see if you like it. Then...head to the stick at the top of this forum to see who has some to offer (usually just for the cost of shipping or free if they're in your area).
We use ours for soaking grains and for smoothies. Pretty much most places that I could use yogurt I use kefir instead.