More specifically, Kombucha is a sweetened tea that is fermented with a SCOBY (a symbiotic colony of bacteria and yeast) to become a nutrient-rich beverage.
The fermentation process takes 7-12 days depending on temperature and the strength of the SCOBY. The SCOBY consumes over 90% of the sugar during fermentation, resulting in a low-sugar finished product. This process is similar to what would happen in sourdough bread or milk/water kefir.
Once a very obscure drink, Kombucha is now an extremely popular beverage among the healthy food community.