AIUI, it replaces the Aurora channel in the Firefox release schedule (which historically went Nightly -> Aurora -> Beta -> Release). In addition, whereas previously all experimental browser features would be held back in Nightly until fully-baked, this new channel will receive experimental developer tools from Nightly while still holding back all experimental non-developer features, so that the browsing experience itself isn't affected.

(Also, it has a darker and more compact visual theme by default that echoes that of the built-in devtools.)