A CPL is a Commercial Pilots Licence
An ATPL is an Air Transport Pilots Licence.
If you have a CPL/IR then you will have what is known as a Frozen ATPL and you may fly for the airlines.
An ATPL is issued when someone obtains the required experience at CPL/IR level. This includes 500 hours Multi Pilot Operations, 1500 Hours Total Time etc.
You cannot be in command of a multi pilot airliner (for instance) without a full ATPL. However you can be a First Officer, ie P2 or P1 U/S with a CPL/IR.
This is a very basic idea of what an ATPL is, I am sure someone else will be able to elaborate on my comments.