I think myself it is mostly convention rather than a real difference.
Fokker 50s are commuter aircraft, so are ATRs.
The difference is not so much in the type of aircraft as in the line flown.
In general, I'd say regional are meant for flights between non-hub fields and commuters for flights between non-hub fields and major cities (usually hubs). The distinction becomes blurry of course if the major city is not a hub...
If the traffic warrants it, a 747 can be used as a commuter or regional aircraft (for example, 747SPs on internal routes in Japan).
I wish I were flying