Here is a great video I found explaining the difference between a Roll Cast and a Switch Cast. This is a better video compared to my previous video on the same subject ( Roll Cast vs Switch Cast ).
Peter Kutzer does a good job explaining the mechanics of the cast. He also added a sub category for the roll cast that he called a dynamic roll cast. Not sure if this is a good idea because to me, differentiating between static roll cast and Switch cast (which is the dynamic form of roll cast) should be good enough and less confusing all round.
As long as the fly & leader does not leave the water during the D-loop set up phase, it is static roll cast. If the fly & leader leave the water momentarily and splash down again to form the anchor and D-loop, then it is a Switch cast.