"Android Auto" (AA) is Googles "Big Thing" for car electronics and is Google's version of Apple's CarPlay.
Android Auto has 2 main hardware components: A headunit in the car, and a phone (or tablet) running the AA companion app. You get in the car, plug your phone in by USB, the phone charges, and you get the AA screen on the HU.
This 2 device model is temporary, and Google's end goal is native Android in the car, with attached/synced phones becoming optional. We will likely hear more on Thursday and Friday at Google IO.
AA is a "special environment" that goes well beyond screen mirroring. It's designed to be non-distracting, but this requires apps to have special AA support, which limits the apps that work, especially at present.
There was a LOT of excitement about AA, but that dissipated quickly when people realized they have to spend at least $500-700 for a compatible headunit, like the Pioneer 4100-NEX, or buy a new car.
I've reverse engineered the Android Auto protocol, and have now released an Android Auto headunit app that runs on tablets, or whatever. Some might find it useful to explore Android Auto capabilities, without buying a new car or a $500+ Pioneer headunut.