What specifically do you want to know about an ICAO ATPL?
In essence there is NO civilian license with words "ICAO ATPL" on it. IT is any license issued by western states such as USA - FAA. Canada - MOT, EU - JAR/JAA, AUS, NZ, HKG a few others.
ATPL stands for Airline Transport Pilot License and is the SAME ticket as an ATP except the Europeans etc tack on the word license (spelt licence) @ the end.
Hope that helps.
From someone who has THREE ICAO ATPLs - FAA, Irish IAA & UK CAA!!