Sugar is used as part of the recipe but it is not used as a sweetener. The sugar is broken down and converted into different components of the finished drink.
The Kombucha culture has been known as a mushroom, a lichen and many other things but the best and most accurate definition we have today is a "symbiotic culture of bacteria and yeast".
Someone compared it to eatting yogart. Yogart is a live culture and is supposed to have endless healthful benefits especially for the gastoinstestinal system I think.
Check out this link. It gives some explanation.
http://www.kombuchatea.co.uk/what-is-kombucha.asp
I'm not completely "sold" on it..but so far...so good!
I'll keep ya posted.