Kombucha is slightly fermented tea. It's very tasty and also extremely healthy. It's made with a yeast culture that ONLY feeds on tea (and dies without it). The culture looks like a mushroom but it's actually yeast (it's called a scoby, though brewers often refer to it as a mushroom). What you do is make a big batch of green tea (or black, but green tastes nicer) with a lot of sugar in it. You let it cool to room temperature (heat kills the scoby) and put it in a big jar with the yeast culture. Then you cover it with a cloth and put it out of the way for about a week. Then you set the scoby aside, bottle the newly-made kombucha (I put in fresh ginger at this point and leave it in the bottle until it gets fizzy).
The yeast digests the sugar and tea. The result is something very healthy. Tea has lots of Good Stuff in it, but our bodies can only absorb a little bit. After it's fermented by this culture, our bodies can absorb ALL the good stuff in the tea. There is also a tiny bit of alcohol, but it's something like 0.05%, so it's not considered an alcoholic beverage. You can keep fermenting it until it's alcoholic, but it would taste like vinegar then.
The problem with kombucha is it became a sort of health food craze at one point and health food stores started selling it in bottles. Do NOT buy this crap. Kombucha is healthy because of the live yeast in it. But if you leave a bottle of kombucha on a shelf, it will keep fermenting. After too many days (maybe 5-7), it starts to taste like vinegar and turn alcoholic. For this reason, all the "kombucha" you see in stores is pasteurized. Pasteurization kills all the yeast so it doesn't ferment anymore, thereby removing all the healthy stuff in the drink. It also changes the taste, so the stuff you would buy in a store usually tastes very yeasty, like bread. It's disgusting.
If you want to try actual kombucha, you have to find someone who makes it. Usually they make more than they can drink (you have to be continually making more or the scoby dies) and will be happy to give you some. Also, the scoby is always growing, and you have to split it now and then or it gets too thick. If you can find someone who makes kombucha, I guarantee they will be happy to give you a spare culture and you can make your own.
It's delicious, really. Especially with ginger in it. Mmmmmmmmm.