It is, in reality, a Symbiotic Colony Of Bacteria and Yeast. A more scientific name for the leathery culture is a "pellicle". Common names are "mother" and "mushroom". It is not a mushroom at all, in fact, technically not even a fungus. It is referred to as a mother because each successful brew produces a "baby" which is seperated from the mother to produce a new brew - or to be passed on to others to start their own brew. This is how it survives, so we are unwittingly part of the symbiotic relationship!