Chromescast is more of a closed wall system that allows talk between a phone/device and itself(Chromecast box) as long as the app has been created for both and it is using the chromecast link/api. Once the android app has told the chromecast app what to do the chromecast box works independently from the android device. IE the android device is free to do other things and is only used as a remote control. The actual stream (Lets say from netflix) is being handled solely from the chromecast.
As of yesterday Google has announced screen sharing between any 4.3+ device and the chromecast even if there is not a chromecast app. Im not sure about sound or how fluid it is, but I'm assuming it will work in a similar to airplay apple.