Chromecast Audio is a small device that plugs into your speaker for streaming music through WiFi. Once set up, simply use your iPhone, iPad, Android phone and tablet, Mac and Windows laptop, or Chromebook to cast your favorite tunes to the best speakers in the house
Basically Chromecast connects through HDMI to your TV and allows media streaming from your phone, tablet or computer. Its primary emphases is made on video streaming. Chromecast Audio on the other hand is made for audio only. It connects directly to your speakers or sound system. Using wireless it casts the audio you send to the speakers.