The CPL is the "Common Public License", an open source license (ie something like the GPL). It was apparently written by IBM but has become better known as the license Microsoft is using for its open-source releases.

The CPL has been OSI approved, meaning that it is a good and useful license. It is not "Shared Source" and should not be confused with it. RMS has said that it is incompatable with the GPL, but the reasons are not all that bad (something about patents) and the next version of the GPL may be modified so this is compatable with that.