<p>In c++ what is object slicing and when does it occur?</p>

Third match in google for "C++ slicing" gives me this Wikipedia article
http://en.wikipedia.org/wiki/Object_slicing
and this (heated, but the first few posts define the problem) :
http://bytes.com/forum/thread163565.html
So it's when you assign an object of a subclass to the super class. The superclass knows nothing of the additional information in the subclass, and hasn't got room to store it, so the additional information gets "sliced off".
If those links don't give enough info for a "good answer" please edit your question to let us know what more you're looking for.