A CPL lets you earn money from flying. If the aircraft you're flying is a multi-pilot aircraft, you are not allowed to be captain. Minimum requirement for getting the license is 18 years of age and 250 hours.
An ATPL adds the privilege of flying as captain of a multi-pilot aircraft for renumeration. Minimum age is 21, and minimum hours is 1500, of which at least 500 must be on a multi-pilot aircraft.
(Those are very simplified JAR requirements - the actual requirements are slightly more detailed, and will vary from country to country.)
No such thing as a lame question - it took me months to figure this out (but that was before I found PPRuNe!)
FFF
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