So just what is Voice User Interface design, anyway?
A VUI or Voice User Interface is, broadly, a thing you can talk to. Or a thing that talks to you. Or both.
These are becoming a lot more common these days. Right now you're probably thinking of Siri, Alexa, or Google Home. These are specifically consumer-facing personal assistants. They can be handy around the house, or for day to day tasks.
"Siri, what's the weather tomorrow?"
"Alexa, turn the bedroom lights on."
"OK Google, open Gmail."
Although VUIs are hot right now, they're not new. Companies use these to support their call centers all the time (often called IVR, or "Interactive Voice Response.") These are tailored to the specific company's business. The best of these systems go far beyond "For this, press 1. For that, press 2" and can understand a lot of different things. They use what's called Natural Language Understanding, and use more open-ended prompts, such as: "How can I help you today? Say something like 'file a claim' or 'make a payment.'" They're also found in the car, and are starting to creep into other consumer electronics, such as the latest Samsung voice-controlled fridge.
There’s also a new term floating around these days: CUI, for Conversational User Interface. This term is a bit broader since it can also refer to text-based conversational user interfaces, such as chat bots. A text conversation is obviously a bit different from a voice conversation, because you hear one, and read the other. But they're not all that different. Both have a similar structure, a give-and-take "flow" if you will. And, both are linear, time-based interactions.