https://www.theverge.com/2018/1/8/16860338/google-assistant-android-auto-support-update-ces-2018

Alongside the bigger news that there will be “smart display” Google Assistant speakers, Google is also announcing that Assistant is going to work in Android Auto. That might be a little confusing to people who have been using Android Auto, because they’ve been saying “OK Google” and issuing commands for directions and music for over a year now.

The difference, you see, is that before it was simply a Google Voice search. Going forward, it will be the actual Google Assistant.