Bitcoin. Cryptocurrencies. Smart contracts. Many people have now heard of the rapidly changing ecosystem of financial technology, but few have wrapped their heads around it. Hundreds of central banks and corporations are incubating a game-changing technology called blockchain—and investors are betting billions on it. Yet only 24 percent of global financial services professionals surveyed in 2017 by PricewaterhouseCoopers (PwC) described themselves as “extremely” or “very” familiar with it. Much of the public is unsure if any of this is legal, if they understand it at all. Evangelists say it has the power to upend entire economic systems; others, such as Emin Gün Sirer, a blockchain researcher at Cornell University, warn that while the technical core is “fascinating and disruptive, there's also a lot of hokum out there.” How to parse the nuance—or get a handle on what a blockchain is?