Note: If you don't have enough mother liquid you can use distilled or non-fermented vinegar (1 tablespoon per liter of brew).
Kombucha is a tea that has been fermented from what's known as a "kombucha colony", a solid mass of microorganisms, primarily acidic bacteria and yeast cultures. 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.
You can brew it at home or buy it bottled from various manufacturers.