Is socket.io the WebRTC or WebSocket or something else?
<p>I'm new to <a href="http://socket.io/" rel="nofollow">socket.io</a>. In Realtime (Web) Applications, we used to choose whether it should be WebRTC or WebSocket (or even SIP, still?) technologies.</p>

<p>What exactly is <code>socket.io</code> in this case please?</p>