A framework that can natively communicate with a browser via mechanisms like Comet or WebSockets. An example is when a new message is posted to an inbox on the server, and your browser immediately updates the message count without polling.
Non-realtime frameworks often have to resort to communicating with websockets over something like zeromq.
Anything that allows two browsers to communicate directly with each other without the user intervening can be called a real-time web framework. Examples include WebSockets, the WebRTC DataChannel, publish/subscribe networks, paid services like PubNub or Kaazing, etc.
