Difference between Linux RPM and DPKG packages

Hello,

Does anyone know what is the exact difference between both packages RPM and DPKG? 

The only thing I found is:

By default two pacakges are available to export.

1) RPM package for Red hat linux clients

2) Debian package for Ubuntu, Cent OS etc.

But did someone try to install for example RPM package on Ubuntu or DPKG on Rat Hat and does it show any errors? Is there really any difference which package on which OS will be installed?