I think TSandPSintheGREEN has asked a very short question that requires a very long answer. Not that I'm about to do that however.
TSandPSintheGREEN, my advice is get familiar with
LASORS and the
Air Navigation Order . Notice I didn't say it would be easy! All the answers to your questions are in there somewhere.
To me personally the most important difference between a CPL and an ATPL is the following clause in the ANO with regards to privilages of a CPL :-
(c) act as pilot in command on a public transport flight of any aeroplane included in Part XII of the licence certificated for single pilot operation;
In other words with a CPL you can't be the Pilot-in-Command of a multi-crew aircraft. That is where the ATPL comes in.
D