No, that's not vinegar you're drinking, it's kombucha. If you've never heard of kombucha it's a tea that has been fermented from what's known as a "kombucha colony." Now, here's the not-so-appealing part: a kombucha colony is a solid mass of microorganisms, primarily acidic bacteria and yeast cultures. It looks not unlike what you might skim off the top of a skanky pond. Mmm, tasty. Basically it's fungus tea.
But I have to say, I love it. It's effervescent, a bit on the sour side, and quite tasty. It's often mixed with other flavors such as fruit juices (like mango and raspberry), is low in calories, and gives you a nice energy boost from the bit of caffeine. It's also reportedly good for your digestive system and aids in liver detoxification. Great for when you've had a bit too much the night before.