Difference between cPCI and ATCA backplanes

Hello,

I am reading up and trying to understand ATCA (Advanced
Telecommunications Computing Architecture). It seems to be defined by
the same group that defined Compact PCI.

What is the difference between the ATCA backplane and a (run of the
mill 6U) cPCI backplane. Can I stick a cPCI board in an ATCA chassis
and vice-versa?

Also what does ATCA provide over cPCI.

Regards,
Xav