Google hasn't had much to say about Android Auto since it previewed the platform back at I/O in June. Now there's some movement as we wait on Android Auto to show up in vehicles. The Google Developers blog has posted an introduction to Android Auto and announced that the final APIs are ready for developers to get to work.
Android Auto requires an Android phone to be plugged in to power the interface. Apps installed on the phone that have support for Android Auto built-in will show up in the Auto interface, but they are displayed in a standard UI to ensure a safe driving experience. So it's definitely more locked down than Android on other platforms. There's a good demo of Android Auto functionality in the video above, which by the way takes place in a car with the apparent code name "Silver Surfer" (it's on the dash).
Developers will be able to make two types of apps for Auto to start—messaging and audio. All the APIs for these apps are ready to go so developers can extend existing apps to work in the car. Now we just need some Android Auto vehicles, and we're good to go.