What is communicated between flight crew and cabin crew during flight?
The FAA at least would seem not to have any requirements of communication between flight and cabin crews. However, airlines must have guidance or standard procedures for such communication. One thing I have always heard--but cannot find in any regulation--is a PA announcement to the effect of "cabin crew, prepare for takeoff;" I can only assume that all airlines have adopted this.

This is not to say that communication does not occur; quite the opposite in fact. Cabin and flight crews communicate regularly about all sorts of issues, from upcoming weather to passenger issues and operational concerns. It is only that such communication is not required by regulations (at least not in the United States).