What is a Neural Network?

They diagnose illnesses, help you find photos of your cat, decide whether to give you a loan1. They make up a huge part of what we call "machine learning" or "artificial intelligence," especially the new, exciting, scary parts of it. And, given the magnitude of the problems they can handle, they're simpler than you'd expect. Most explanations of neural networks for lay people are kind of lousy, because they always start by talking about brains. They'll define neural networks as "computer systems modeled on the human brain" or "programs that learn the same way as people" or even as "artificial brains."