What is Yeast?

In this podcast we've been discussing fermentation as a verb, almost comparing it to an erupting volcano. We've explored the many was in which agave and even coffee are transformed by this process but we haven't really explored yeast in detail. What exactly is it? How does it survive? Are there better yeasts than others? Can different yeasts create distinct flavors? I can't claim that we came up with clear-cut answers to all of these questions. But we definitely tried our best. Please welcome again Cesar Ojeda and his infinite curiosity for all things fermentation.