What is the difference between Catia V5 and Catia V6?

The majority of the difference between CATIA V5 and V6 doesn't lie with the design tools themselves, the interface in the design workbenches is virtually identical, plus a few new workbenches and enhancements.
It can be modified in the same way as V5.
The big differences lie in the data management side.
CATIA V6 requires an ENOVIA database for it to connect to, with database searches and linking all managed within that database and accessed directly from CATIA, rather than some other data management program (There are no more local files).
It might sound a bit onerous to start with, but it solves the various headaches associated with linking documents together in various ways in a file/folder system (No more hours spent in the Desk command recreating broken links for a start).