Kombucha is fermented tea. The flavor of kombucha and its health-promoting properties come from a microbial culture ('the mother'). This culture produces organic acids, B-vitamines and enzymes.
Kombucha comes from Asia where is has been consumed for thousands of years. Just like tea, kombucha most likely originated in China where it was praised for its detoxicating properties.
In the early 20th century a lot of research was conducted in Russia. Russian soldiers received kombucha during the 2nd World War to keep their stomach and digestion in good health. In 1951, the Russian Academy of Science in Moscow published a population study in which it described increased cancer resistance from daily intake of kombucha.
Scientific studies have been rare in recent years, but there is some evidence to support the health claims made in traditional medicine. According to a study from 2008, kombucha has 'excellent anti-oxidative properties'. The high content of antioxidants in kombucha has been linked to anti-inflammatory properties and an increased immune defence.
If you are interested in the science of kombucha here is a selection of scientific publications. Researchers have looked into kombucha's antioxidant properties, its antimicrobial properties, its mineral and vitamin content, its enzymes content, or its protective properties. Some publications also provide an overview of health claims and scientific evidence, such as this review. Much more information can easily be found on the web, however some sources are more credible than others.