The Nexus Q is a cloud connected jukebox. It works as a sort of receiver for Google Play connected devices, allowing anyone with access to Google Music or YouTube, as just two examples, to play music or videos on whatever the Nexus Q is hooked up to.
If you have your Nexus Q hooked up to a set a speakers (like the $399 pair being sold on Google Play), anyone who connects their device to that Nexus Q can play music through those speakers. No need to pass around phones or connect to different accounts.
As far as the tech specs, the Nexus Q features optical audio, an ethernet port, micro HDMI port, micro USB and banana jack speaker outputs, all wrapped in a solid black ball shaped casing, surrounded by a ring of 32 color changing LED lights.