The difference between AI, machine learning and digital assistants

We hear a lot about artificial intelligence (AI), machine learning and digital assistants these days. But what does it all mean? Microsoft, Google, Amazon, Apple, Facebook and others are aggressively imbuing their products and services with "intelligence." We're told that computer systems will become smarter, will anticipate our needs, proactively serve us, and understand us in various contexts and via multiple methods including voice, text, inking and motion. The digital "face" of this promising tech comes in the form of assistants like Cortana, Assistant, Siri, Alexa, and Bixby.