This a simple design, rounded so you can connect to your old speakers and make them "connected". The small round object has a support for 3.5mm connector, RCA or Optical which should be enough for any kind of streaming. There is a mandatory micro USB port on the device to power it.
The way that this device is supposed to work, you will connect it to the speaker, press a cast icon on your android phone or tablet, and the music should start right away. Google ships the device with a 3.5mm analogue stereo patch cable, power cable and adapter. You will have to buy a third parity cable if you need the RCA or optical cable. The device comes with a 3.5mm jack fore analogue and optical digital audio output. Chromecast Audio measures 51.9 x 51.9 x 13.49 mm and weights 30.7 g (0.068 lbs). Its power cable is 1.75 m long while the Audio cable is a rather short 146 mm or 5.74 inches.
The device needs a Wi-Fi and stream to work. It supports Wi-Fi 802.11ac in both 2.4GHz and 5 GHz variants.
Chromecast Audio will work with a computer or mobile device powered with Android 4.1+, iOS 7.0+, Windows 7, or Mac OS X 10.7. It sells for $35 for Americans and starts at €39 if you are based in Europe.
If you have older speakers, this device will turn them to connected speakers right away, and you won't have to go and buy some ridiculous overpriced connected speakers like Sonos.
It was out of stock in Europe when we checked.